Transformer Tutorial

Author: Zihao Ye, Jinjing Zhou, Qipeng Guo, Quan Gan, Zheng Zhang

The Transformer model, as a replacement of CNN/RNN architecture for sequence modeling, was introduced in Google’s paper: Attention is All You Need. It improved the state of the art for machine translation as well as natural language inference task (GPT). Recent work on pre-training Transformer with large scale corpus (BERT) supports that it is capable of learning high quality semantic representation.

The interesting part of Transformer is its extensive employment of attention. The classic use of attention comes from machine translation model, where the output token attends to all input tokens.

Transformer additionally applies self-attention in both decoder and encoder. This process forces words relate to each other to combine together, irrespective of their positions in the sequence. This is different from RNN-based model, where words (in the source sentence) are combined along the chain, which is thought to be too constrained.

Attention Layer of Transformer

In Attention Layer of Transformer, for each node the module learns to assign weights on its in-coming edges. For node pair \((i, j)\) (from \(i\) to \(j\)) with node \(x_i, x_j \in \mathbb{R}^n\), the score of their connection is defined as follows:

\[\begin{split}q_j = W_q\cdot x_j \\ k_i = W_k\cdot x_i\\ v_i = W_v\cdot x_i\\ \textrm{score} = q_j^T k_i\end{split}\]

where \(W_q, W_k, W_v \in \mathbb{R}^{n\times d_k}\) map the representations \(x\) to “query”, “key”, and “value” space respectively.

There are other possibilities to implement the score function. The dot product measures the similarity of a given query \(q_j\) and a key \(k_i\): if \(j\) needs the information stored in \(i\), the query vector at position \(j\) (\(q_j\)) is supposed to be close to key vector at position \(i\) (\(k_i\)).

The score is then used to compute the sum of the incoming values, normalized over the weights of edges, stored in \(\textrm{wv}\). Then we apply an affine layer to \(\textrm{wv}\) to get the output \(o\):

\[\begin{split}w_{ji} = \frac{\exp\{\textrm{score}_{ji} \}}{\sum\limits_{(k, i)\in E}\exp\{\textrm{score}_{ki} \}} \\ \textrm{wv}_i = \sum_{(k, i)\in E} w_{ki} v_k \\ o = W_o\cdot \textrm{wv} \\\end{split}\]

Multi-Head Attention Layer

In Transformer, attention is multi-headed. A head is very much like a channel in a convolutional network. The Multi-Head Attention consist of multiple attention heads, in which each head refers to a single attention module. \(\textrm{wv}^{(i)}\) for all the heads are concatenated and mapped to output \(o\) with an affine layer:

\[o = W_o \cdot \textrm{concat}\left(\left[\textrm{wv}^{(0)}, \textrm{wv}^{(1)}, \cdots, \textrm{wv}^{(h)}\right]\right)\]

The code below wraps necessary components for Multi-Head Attention, and provide two interfaces:

  • get maps state ‘x’, to query, key and value, which is required by following steps(propagate_attention).
  • get_o maps the updated value after attention to the output \(o\) for post-processing.
class MultiHeadAttention(nn.Module):
    "Multi-Head Attention"
    def __init__(self, h, dim_model):
        "h: number of heads; dim_model: hidden dimension"
        super(MultiHeadAttention, self).__init__()
        self.d_k = dim_model // h
        self.h = h
        # W_q, W_k, W_v, W_o
        self.linears = clones(nn.Linear(dim_model, dim_model), 4)

    def get(self, x, fields='qkv'):
        "Return a dict of queries / keys / values."
        batch_size = x.shape[0]
        ret = {}
        if 'q' in fields:
            ret['q'] = self.linears[0](x).view(batch_size, self.h, self.d_k)
        if 'k' in fields:
            ret['k'] = self.linears[1](x).view(batch_size, self.h, self.d_k)
        if 'v' in fields:
            ret['v'] = self.linears[2](x).view(batch_size, self.h, self.d_k)
        return ret

    def get_o(self, x):
        "get output of the multi-head attention"
        batch_size = x.shape[0]
        return self.linears[3](x.view(batch_size, -1))

In this tutorial, we show a simplified version of the implementation in order to highlight the most important design points (for instance we only show single-head attention); the complete code can be found here. The overall structure is similar to the one from The Annotated Transformer.

How DGL Implements Transformer with a Graph Neural Network

We offer a different perspective of Transformer by treating the attention as edges in a graph and adopt message passing on the edges to induce the appropriate processing.

Graph Structure

We construct the graph by mapping tokens of the source and target sentence to nodes. The complete Transformer graph is made up of three subgraphs:

Source language graph. This is a complete graph, each token \(s_i\) can attend to any other token \(s_j\) (including self-loops). image0 Target language graph. The graph is half-complete, in that \(t_i\) attends only to \(t_j\) if \(i > j\) (an output token can not depend on future words). image1 Cross-language graph. This is a bi-partitie graph, where there is an edge from every source token \(s_i\) to every target token \(t_j\), meaning every target token can attend on source tokens. image2

The full picture looks like this: image3

We pre-build the graphs in dataset preparation stage.

Message Passing

Once we defined the graph structure, we can move on to defining the computation for message passing.

Assuming that we have already computed all the queries \(q_i\), keys \(k_i\) and values \(v_i\). For each node \(i\) (no matter whether it is a source token or target token), we can decompose the attention computation into two steps:

  1. Message computation: Compute attention score \(\mathrm{score}_{ij}\) between \(i\) and all nodes \(j\) to be attended over, by taking the scaled-dot product between \(q_i\) and \(k_j\). The message sent from \(j\) to \(i\) will consist of the score \(\mathrm{score}_{ij}\) and the value \(v_j\).
  2. Message aggregation: Aggregate the values \(v_j\) from all \(j\) according to the scores \(\mathrm{score}_{ij}\).

Naive Implementation

Message Computation

Compute score and send source node’s v to destination’s mailbox

def message_func(edges):
    return {'score': ((edges.src['k'] * edges.dst['q'])
                      .sum(-1, keepdim=True)),
            'v': edges.src['v']}
Message Aggregation

Normalize over all in-edges and weighted sum to get output

import torch as th
import torch.nn.functional as F

def reduce_func(nodes, d_k=64):
    v = nodes.mailbox['v']
    att = F.softmax(nodes.mailbox['score'] / th.sqrt(d_k), 1)
    return {'dx': (att * v).sum(1)}
Execute on specific edges
import functools.partial as partial
def naive_propagate_attention(self, g, eids):
    g.send_and_recv(eids, message_func, partial(reduce_func, d_k=self.d_k))

Speeding up with built-in functions

To speed up the message passing process, we utilize DGL’s builtin function, including:

  • fn.src_mul_egdes(src_field, edges_field, out_field) multiplies source’s attribute and edges attribute, and send the result to the destination node’s mailbox keyed by out_field.
  • fn.copy_edge(edges_field, out_field) copies edge’s attribute to destination node’s mailbox.
  • fn.sum(edges_field, out_field) sums up edge’s attribute and sends aggregation to destination node’s mailbox.

Here we assemble those built-in function into propagate_attention, which is also the main graph operation function in our final implementation. To accelerate, we break the softmax operation into the following steps. Recall that for each head there are two phases:

  1. Compute attention score by multiply src node’s k and dst node’s q

    • g.apply_edges(src_dot_dst('k', 'q', 'score'), eids)
  2. Scaled Softmax over all dst nodes’ in-coming edges

    • Step 1: Exponentialize score with scale normalize constant

      • g.apply_edges(scaled_exp('score', np.sqrt(self.d_k)))

        \[\textrm{score}_{ij}\leftarrow\exp{\left(\frac{\textrm{score}_{ij}}{ \sqrt{d_k}}\right)}\]
    • Step 2: Get the “values” on associated nodes weighted by “scores” on in-coming edges of each node; get the sum of “scores” on in-coming edges of each node for normalization. Note that here \(\textrm{wv}\) is not normalized.

      • msg: fn.src_mul_edge('v', 'score', 'v'), reduce: fn.sum('v', 'wv')

        \[\textrm{wv}_j=\sum_{i=1}^{N} \textrm{score}_{ij} \cdot v_i\]
      • msg: fn.copy_edge('score', 'score'), reduce: fn.sum('score', 'z')

        \[\textrm{z}_j=\sum_{i=1}^{N} \textrm{score}_{ij}\]

The normalization of \(\textrm{wv}\) is left to post processing.

def src_dot_dst(src_field, dst_field, out_field):
    def func(edges):
        return {out_field: (edges.src[src_field] * edges.dst[dst_field]).sum(-1, keepdim=True)}

    return func

def scaled_exp(field, scale_constant):
    def func(edges):
        # clamp for softmax numerical stability
        return {field: th.exp((edges.data[field] / scale_constant).clamp(-5, 5))}

    return func


def propagate_attention(self, g, eids):
    # Compute attention score
    g.apply_edges(src_dot_dst('k', 'q', 'score'), eids)
    g.apply_edges(scaled_exp('score', np.sqrt(self.d_k)))
    # Update node state
    g.send_and_recv(eids,
                    [fn.src_mul_edge('v', 'score', 'v'), fn.copy_edge('score', 'score')],
                    [fn.sum('v', 'wv'), fn.sum('score', 'z')])

Preprocessing and Postprocessing

In Transformer, data needs to be pre- and post-processed before and after the propagate_attention function.

Preprocessing The preprocessing function pre_func first normalizes the node representations and then map them to a set of queries, keys and values, using self-attention as an example:

\[\begin{split}x \leftarrow \textrm{LayerNorm}(x) \\ [q, k, v] \leftarrow [W_q, W_k, W_v ]\cdot x\end{split}\]

Postprocessing The postprocessing function post_funcs completes the whole computation correspond to one layer of the transformer: 1. Normalize \(\textrm{wv}\) and get the output of Multi-Head Attention Layer \(o\).

\[\begin{split}\textrm{wv} \leftarrow \frac{\textrm{wv}}{z} \\ o \leftarrow W_o\cdot \textrm{wv} + b_o\end{split}\]

add residual connection:

\[x \leftarrow x + o\]
  1. Applying a two layer position-wise feed forward layer on \(x\) then add residual connection:

    \[x \leftarrow x + \textrm{LayerNorm}(\textrm{FFN}(x))\]

    where \(\textrm{FFN}\) refers to the feed forward function.

class Encoder(nn.Module):
    def __init__(self, layer, N):
        super(Encoder, self).__init__()
        self.N = N
        self.layers = clones(layer, N)
        self.norm = LayerNorm(layer.size)

    def pre_func(self, i, fields='qkv'):
        layer = self.layers[i]
        def func(nodes):
            x = nodes.data['x']
            norm_x = layer.sublayer[0].norm(x)
            return layer.self_attn.get(norm_x, fields=fields)
        return func

    def post_func(self, i):
        layer = self.layers[i]
        def func(nodes):
            x, wv, z = nodes.data['x'], nodes.data['wv'], nodes.data['z']
            o = layer.self_attn.get_o(wv / z)
            x = x + layer.sublayer[0].dropout(o)
            x = layer.sublayer[1](x, layer.feed_forward)
            return {'x': x if i < self.N - 1 else self.norm(x)}
        return func

class Decoder(nn.Module):
    def __init__(self, layer, N):
        super(Decoder, self).__init__()
        self.N = N
        self.layers = clones(layer, N)
        self.norm = LayerNorm(layer.size)

    def pre_func(self, i, fields='qkv', l=0):
        layer = self.layers[i]
        def func(nodes):
            x = nodes.data['x']
            if fields == 'kv':
                norm_x = x # In enc-dec attention, x has already been normalized.
            else:
                norm_x = layer.sublayer[l].norm(x)
            return layer.self_attn.get(norm_x, fields)
        return func

    def post_func(self, i, l=0):
        layer = self.layers[i]
        def func(nodes):
            x, wv, z = nodes.data['x'], nodes.data['wv'], nodes.data['z']
            o = layer.self_attn.get_o(wv / z)
            x = x + layer.sublayer[l].dropout(o)
            if l == 1:
                x = layer.sublayer[2](x, layer.feed_forward)
            return {'x': x if i < self.N - 1 else self.norm(x)}
        return func

This completes all procedures of one layer of encoder and decoder in Transformer.

Note

The sublayer connection part is little bit different from the original paper. However our implementation is the same as The Annotated Transformer and OpenNMT.

Main class of Transformer Graph

The processing flow of Transformer can be seen as a 2-stage message-passing within the complete graph (adding pre- and post- processing appropriately): 1) self-attention in encoder, 2) self-attention in decoder followed by cross-attention between encoder and decoder, as shown below. image4

class Transformer(nn.Module):
    def __init__(self, encoder, decoder, src_embed, tgt_embed, pos_enc, generator, h, d_k):
        super(Transformer, self).__init__()
        self.encoder, self.decoder = encoder, decoder
        self.src_embed, self.tgt_embed = src_embed, tgt_embed
        self.pos_enc = pos_enc
        self.generator = generator
        self.h, self.d_k = h, d_k

    def propagate_attention(self, g, eids):
        # Compute attention score
        g.apply_edges(src_dot_dst('k', 'q', 'score'), eids)
        g.apply_edges(scaled_exp('score', np.sqrt(self.d_k)))
        # Send weighted values to target nodes
        g.send_and_recv(eids,
                        [fn.src_mul_edge('v', 'score', 'v'), fn.copy_edge('score', 'score')],
                        [fn.sum('v', 'wv'), fn.sum('score', 'z')])

    def update_graph(self, g, eids, pre_pairs, post_pairs):
        "Update the node states and edge states of the graph."

        # Pre-compute queries and key-value pairs.
        for pre_func, nids in pre_pairs:
            g.apply_nodes(pre_func, nids)
        self.propagate_attention(g, eids)
        # Further calculation after attention mechanism
        for post_func, nids in post_pairs:
            g.apply_nodes(post_func, nids)

    def forward(self, graph):
        g = graph.g
        nids, eids = graph.nids, graph.eids

        # Word Embedding and Position Embedding
        src_embed, src_pos = self.src_embed(graph.src[0]), self.pos_enc(graph.src[1])
        tgt_embed, tgt_pos = self.tgt_embed(graph.tgt[0]), self.pos_enc(graph.tgt[1])
        g.nodes[nids['enc']].data['x'] = self.pos_enc.dropout(src_embed + src_pos)
        g.nodes[nids['dec']].data['x'] = self.pos_enc.dropout(tgt_embed + tgt_pos)

        for i in range(self.encoder.N):
            # Step 1: Encoder Self-attention
            pre_func = self.encoder.pre_func(i, 'qkv')
            post_func = self.encoder.post_func(i)
            nodes, edges = nids['enc'], eids['ee']
            self.update_graph(g, edges, [(pre_func, nodes)], [(post_func, nodes)])

        for i in range(self.decoder.N):
            # Step 2: Dncoder Self-attention
            pre_func = self.decoder.pre_func(i, 'qkv')
            post_func = self.decoder.post_func(i)
            nodes, edges = nids['dec'], eids['dd']
            self.update_graph(g, edges, [(pre_func, nodes)], [(post_func, nodes)])
            # Step 3: Encoder-Decoder attention
            pre_q = self.decoder.pre_func(i, 'q', 1)
            pre_kv = self.decoder.pre_func(i, 'kv', 1)
            post_func = self.decoder.post_func(i, 1)
            nodes_e, nodes_d, edges = nids['enc'], nids['dec'], eids['ed']
            self.update_graph(g, edges, [(pre_q, nodes_d), (pre_kv, nodes_e)], [(post_func, nodes_d)])

        return self.generator(g.ndata['x'][nids['dec']])

Note

By calling update_graph function, we can “DIY our own Transformer” on any subgraphs with nearly the same code. This flexibility enables us to discover new, sparse structures (c.f. local attention mentioned here). Note in our implementation we does not use mask or padding, which makes the logic more clear and saves memory. The trade-off is that the implementation is slower; we will improve with future DGL optimizations.

Training

This tutorial does not cover several other techniques such as Label Smoothing and Noam Optimizations mentioned in the original paper. For detailed description about these modules, we recommend you to read The Annotated Transformer written by Harvard NLP team.

Task and the Dataset

The Transformer is a general framework for a variety of NLP tasks. In this tutorial we only focus on the sequence to sequence learning: it’s a typical case to illustrate how it works.

As for the dataset, we provide two toy tasks: copy and sort, together with two real-world translation tasks: multi30k en-de task and wmt14 en-de task.

  • copy dataset: copy input sequences to output. (train/valid/test: 9000, 1000, 1000)
  • sort dataset: sort input sequences as output. (train/valid/test: 9000, 1000, 1000)
  • Multi30k en-de, translate sentences from En to De. (train/valid/test: 29000, 1000, 1000)
  • WMT14 en-de, translate sentences from En to De. (Train/Valid/Test: 4500966/3000/3003)

Note

We are working on training with wmt14, which requires Multi-GPU support(this would be fixed soon).

Graph Building

Batching Just like how we handle Tree-LSTM. We build a graph pool in advance, including all possible combination of input lengths and output lengths. Then for each sample in a batch, we call dgl.batch to batch graphs of their sizes together in to a single large graph.

We have wrapped the process of creating graph pool and building BatchedGraph in dataset.GraphPool and dataset.TranslationDataset.

graph_pool = GraphPool()

data_iter = dataset(graph_pool, mode='train', batch_size=1, devices=devices)
for graph in data_iter:
    print(graph.nids['enc']) # encoder node ids
    print(graph.nids['dec']) # decoder node ids
    print(graph.eids['ee']) # encoder-encoder edge ids
    print(graph.eids['ed']) # encoder-decoder edge ids
    print(graph.eids['dd']) # decoder-decoder edge ids
    print(graph.src[0]) # Input word index list
    print(graph.src[1]) # Input positions
    print(graph.tgt[0]) # Output word index list
    print(graph.tgt[1]) # Ouptut positions
    break

Output:

tensor([0, 1, 2, 3, 4, 5, 6, 7, 8], device='cuda:0')
tensor([ 9, 10, 11, 12, 13, 14, 15, 16, 17, 18], device='cuda:0')
tensor([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16, 17,
        18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,
        36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53,
        54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71,
        72, 73, 74, 75, 76, 77, 78, 79, 80], device='cuda:0')
tensor([ 81,  82,  83,  84,  85,  86,  87,  88,  89,  90,  91,  92,  93,  94,
         95,  96,  97,  98,  99, 100, 101, 102, 103, 104, 105, 106, 107, 108,
        109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122,
        123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136,
        137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150,
        151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164,
        165, 166, 167, 168, 169, 170], device='cuda:0')
tensor([171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184,
        185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198,
        199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212,
        213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225],
       device='cuda:0')
tensor([28, 25,  7, 26,  6,  4,  5,  9, 18], device='cuda:0')
tensor([0, 1, 2, 3, 4, 5, 6, 7, 8], device='cuda:0')
tensor([ 0, 28, 25,  7, 26,  6,  4,  5,  9, 18], device='cuda:0')
tensor([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], device='cuda:0')

Put it all together

We train a one-head transformer with one layer, 128 dimension on copy task. Other parameters are set to default.

Note that we do not involve inference module in this tutorial (which requires beam search), please refer to the Github Repo for full implementation.

from tqdm import tqdm
import torch as th
import numpy as np

from loss import LabelSmoothing, SimpleLossCompute
from modules import make_model
from optims import NoamOpt
from dgl.contrib.transformer import get_dataset, GraphPool

def run_epoch(data_iter, model, loss_compute, is_train=True):
    for i, g in tqdm(enumerate(data_iter)):
        with th.set_grad_enabled(is_train):
            output = model(g)
            loss = loss_compute(output, g.tgt_y, g.n_tokens)
    print('average loss: {}'.format(loss_compute.avg_loss))
    print('accuracy: {}'.format(loss_compute.accuracy))

N = 1
batch_size = 128
devices = ['cuda' if th.cuda.is_available() else 'cpu']

dataset = get_dataset("copy")
V = dataset.vocab_size
criterion = LabelSmoothing(V, padding_idx=dataset.pad_id, smoothing=0.1)
dim_model = 128

# Create model
model = make_model(V, V, N=N, dim_model=128, dim_ff=128, h=1)

# Sharing weights between Encoder & Decoder
model.src_embed.lut.weight = model.tgt_embed.lut.weight
model.generator.proj.weight = model.tgt_embed.lut.weight

model, criterion = model.to(devices[0]), criterion.to(devices[0])
model_opt = NoamOpt(dim_model, 1, 400,
                    th.optim.Adam(model.parameters(), lr=1e-3, betas=(0.9, 0.98), eps=1e-9))
loss_compute = SimpleLossCompute

att_maps = []
for epoch in range(4):
    train_iter = dataset(graph_pool, mode='train', batch_size=batch_size, devices=devices)
    valid_iter = dataset(graph_pool, mode='valid', batch_size=batch_size, devices=devices)
    print('Epoch: {} Training...'.format(epoch))
    model.train(True)
    run_epoch(train_iter, model,
              loss_compute(criterion, model_opt), is_train=True)
    print('Epoch: {} Evaluating...'.format(epoch))
    model.att_weight_map = None
    model.eval()
    run_epoch(valid_iter, model,
              loss_compute(criterion, None), is_train=False)
    att_maps.append(model.att_weight_map)

Visualization

After training, we can visualize the attention our Transformer generates on copy task:

src_seq = dataset.get_seq_by_id(VIZ_IDX, mode='valid', field='src')
tgt_seq = dataset.get_seq_by_id(VIZ_IDX, mode='valid', field='tgt')[:-1]
# visualize head 0 of encoder-decoder attention
att_animation(att_maps, 'e2d', src_seq, tgt_seq, 0)

image5 from the figure we see the decoder nodes gradually learns to attend to corresponding nodes in input sequence, which is the expected behavior.

Multi-Head Attention

Besides the attention of a one-head attention trained on toy task. We also visualize the attention scores of Encoder’s Self Attention, Decoder’s Self Attention and the Encoder-Decoder attention of an one-Layer Transformer network trained on multi-30k dataset.

From the visualization we observe the diversity of different heads (which is what we expected: different heads learn different relations between word pairs):

  • Encoder Self-Attention image6
  • Encoder-Decoder Attention Most words in target sequence attend on their related words in source sequence, for example: when generating “See” (in De), several heads attend on “lake”; when generating “Eisfischerhütte”, several heads attend on “ice”. image7
  • Decoder Self-Attention Most words attend on their previous few words. image8

Adaptive Universal Transformer

A recent paper by Google: Universal Transformer is an example to show how update_graph adapts to more complex updating rules.

The Universal Transformer was proposed to address the problem that vanilla Transformer is not computationally universal by introducing recurrence in Transformer:

  • The basic idea of Universal Transformer is to repeatedly revise its representations of all symbols in the sequence with each recurrent step by applying a Transformer layer on the representations.
  • Compared to vanilla Transformer, Universal Transformer shares weights among its layers, and it does not fix the recurrence time (which means the number of layers in Transformer).

A further optimization employs an adaptive computation time (ACT) mechanism to allow the model to dynamically adjust the number of times the representation of each position in a sequence is revised (refereed to as step hereinafter). This model is also known as the Adaptive Universal Transformer(referred to as AUT hereinafter).

In AUT, we maintain an “active nodes” list. In each step \(t\), we compute a halting probability: \(h (0<h<1)\) for all nodes in this list by:

\[h^t_i = \sigma(W_h x^t_i + b_h)\]

then dynamically decide which nodes are still active. A node is halted at time \(T\) if and only if \(\sum_{t=1}^{T-1} h_t < 1 - \varepsilon \leq \sum_{t=1}^{T}h_t\). Halted nodes are removed from the list. The procedure proceeds until the list is empty or a pre-defined maximum step is reached. From DGL’s perspective, this means that the “active” graph becomes sparser over time.

The final state of a node \(s_i\) is a weighted average of \(x_i^t\) by \(h_i^t\):

\[s_i = \sum_{t=1}^{T} h_i^t\cdot x_i^t\]

In DGL, the algorithm is easy to implement, we just need to call update_graph on nodes that are still active and edges associated with this nodes. The following code shows the Universal Transformer class in DGL:

class UTransformer(nn.Module):
    "Universal Transformer(https://arxiv.org/pdf/1807.03819.pdf) with ACT(https://arxiv.org/pdf/1603.08983.pdf)."
    MAX_DEPTH = 8
    thres = 0.99
    act_loss_weight = 0.01
    def __init__(self, encoder, decoder, src_embed, tgt_embed, pos_enc, time_enc, generator, h, d_k):
        super(UTransformer, self).__init__()
        self.encoder,  self.decoder = encoder, decoder
        self.src_embed, self.tgt_embed = src_embed, tgt_embed
        self.pos_enc, self.time_enc = pos_enc, time_enc
        self.halt_enc = HaltingUnit(h * d_k)
        self.halt_dec = HaltingUnit(h * d_k)
        self.generator = generator
        self.h, self.d_k = h, d_k

    def step_forward(self, nodes):
        # add positional encoding and time encoding, increment step by one
        x = nodes.data['x']
        step = nodes.data['step']
        pos = nodes.data['pos']
        return {'x': self.pos_enc.dropout(x + self.pos_enc(pos.view(-1)) + self.time_enc(step.view(-1))),
                'step': step + 1}

    def halt_and_accum(self, name, end=False):
        "field: 'enc' or 'dec'"
        halt = self.halt_enc if name == 'enc' else self.halt_dec
        thres = self.thres
        def func(nodes):
            p = halt(nodes.data['x'])
            sum_p = nodes.data['sum_p'] + p
            active = (sum_p < thres) & (1 - end)
            _continue = active.float()
            r = nodes.data['r'] * (1 - _continue) + (1 - sum_p) * _continue
            s = nodes.data['s'] + ((1 - _continue) * r + _continue * p) * nodes.data['x']
            return {'p': p, 'sum_p': sum_p, 'r': r, 's': s, 'active': active}
        return func

    def propagate_attention(self, g, eids):
        # Compute attention score
        g.apply_edges(src_dot_dst('k', 'q', 'score'), eids)
        g.apply_edges(scaled_exp('score', np.sqrt(self.d_k)), eids)
        # Send weighted values to target nodes
        g.send_and_recv(eids,
                        [fn.src_mul_edge('v', 'score', 'v'), fn.copy_edge('score', 'score')],
                        [fn.sum('v', 'wv'), fn.sum('score', 'z')])

    def update_graph(self, g, eids, pre_pairs, post_pairs):
        "Update the node states and edge states of the graph."
        # Pre-compute queries and key-value pairs.
        for pre_func, nids in pre_pairs:
            g.apply_nodes(pre_func, nids)
        self.propagate_attention(g, eids)
        # Further calculation after attention mechanism
        for post_func, nids in post_pairs:
            g.apply_nodes(post_func, nids)

    def forward(self, graph):
        g = graph.g
        N, E = graph.n_nodes, graph.n_edges
        nids, eids = graph.nids, graph.eids

        # embed & pos
        g.nodes[nids['enc']].data['x'] = self.src_embed(graph.src[0])
        g.nodes[nids['dec']].data['x'] = self.tgt_embed(graph.tgt[0])
        g.nodes[nids['enc']].data['pos'] = graph.src[1]
        g.nodes[nids['dec']].data['pos'] = graph.tgt[1]

        # init step
        device = next(self.parameters()).device
        g.ndata['s'] = th.zeros(N, self.h * self.d_k, dtype=th.float, device=device)    # accumulated state
        g.ndata['p'] = th.zeros(N, 1, dtype=th.float, device=device)                    # halting prob
        g.ndata['r'] = th.ones(N, 1, dtype=th.float, device=device)                     # remainder
        g.ndata['sum_p'] = th.zeros(N, 1, dtype=th.float, device=device)                # sum of pondering values
        g.ndata['step'] = th.zeros(N, 1, dtype=th.long, device=device)                  # step
        g.ndata['active'] = th.ones(N, 1, dtype=th.uint8, device=device)                # active

        for step in range(self.MAX_DEPTH):
            pre_func = self.encoder.pre_func('qkv')
            post_func = self.encoder.post_func()
            nodes = g.filter_nodes(lambda v: v.data['active'].view(-1), nids['enc'])
            if len(nodes) == 0: break
            edges = g.filter_edges(lambda e: e.dst['active'].view(-1), eids['ee'])
            end = step == self.MAX_DEPTH - 1
            self.update_graph(g, edges,
                              [(self.step_forward, nodes), (pre_func, nodes)],
                              [(post_func, nodes), (self.halt_and_accum('enc', end), nodes)])

        g.nodes[nids['enc']].data['x'] = self.encoder.norm(g.nodes[nids['enc']].data['s'])

        for step in range(self.MAX_DEPTH):
            pre_func = self.decoder.pre_func('qkv')
            post_func = self.decoder.post_func()
            nodes = g.filter_nodes(lambda v: v.data['active'].view(-1), nids['dec'])
            if len(nodes) == 0: break
            edges = g.filter_edges(lambda e: e.dst['active'].view(-1), eids['dd'])
            self.update_graph(g, edges,
                              [(self.step_forward, nodes), (pre_func, nodes)],
                              [(post_func, nodes)])

            pre_q = self.decoder.pre_func('q', 1)
            pre_kv = self.decoder.pre_func('kv', 1)
            post_func = self.decoder.post_func(1)
            nodes_e = nids['enc']
            edges = g.filter_edges(lambda e: e.dst['active'].view(-1), eids['ed'])
            end = step == self.MAX_DEPTH - 1
            self.update_graph(g, edges,
                              [(pre_q, nodes), (pre_kv, nodes_e)],
                              [(post_func, nodes), (self.halt_and_accum('dec', end), nodes)])

        g.nodes[nids['dec']].data['x'] = self.decoder.norm(g.nodes[nids['dec']].data['s'])
        act_loss = th.mean(g.ndata['r']) # ACT loss

        return self.generator(g.ndata['x'][nids['dec']]), act_loss * self.act_loss_weight

Here we call filter_nodes and filter_edge to find nodes/edges that are still active:

Note

  • filter_nodes() takes a predicate and a node ID list/tensor as input, then returns a tensor of node IDs that satisfy the given predicate.
  • filter_edges() takes a predicate and an edge ID list/tensor as input, then returns a tensor of edge IDs that satisfy the given predicate.

for the full implementation, please refer to our Github Repo.

The figure below shows the effect of Adaptive Computational Time(different positions of a sentence were revised different times):

image9

We also visualize the dynamics of step distribution on nodes during the training of AUT on sort task(reach 99.7% accuracy), which demonstrates how AUT learns to reduce recurrence steps during training. image10

Note

We apologize that this notebook itself is not runnable due to many dependencies, please download the 7_transformer.py, and copy the python script to directory examples/pytorch/transformer then run python 7_transformer.py to see how it works.

Total running time of the script: ( 0 minutes 0.076 seconds)

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