"""
.. _model-transformer:
Transformer as a Graph Neural Network
======================================
**Author**: Zihao Ye, Jinjing Zhou, Qipeng Guo, Quan Gan, Zheng Zhang
.. warning::
The tutorial aims at gaining insights into the paper, with code as a mean
of explanation. The implementation thus is NOT optimized for running
efficiency. For recommended implementation, please refer to the `official
examples `_.
"""
################################################################################################
# In this tutorial, you learn about a simplified implementation of the Transformer model.
# You can see highlights of the most important design points. For instance, there is
# only single-head attention. The complete code can be found
# `here `__.
#
# The overall structure is similar to the one from the research papaer `Annotated
# Transformer `__.
#
# The Transformer model, as a replacement of CNN/RNN architecture for
# sequence modeling, was introduced in the research 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 the attention layer of Transformer, for each node the module learns to
# assign weights on its in-coming edges. For node pair :math:`(i, j)`
# (from :math:`i` to :math:`j`) with node
# :math:`x_i, x_j \in \mathbb{R}^n`, the score of their connection is
# defined as follows:
#
# .. math::
#
#
# 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
#
# where :math:`W_q, W_k, W_v \in \mathbb{R}^{n\times d_k}` map the
# representations :math:`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 :math:`q_j` and a key
# :math:`k_i`: if :math:`j` needs the information stored in :math:`i`, the
# query vector at position :math:`j` (:math:`q_j`) is supposed to be close
# to key vector at position :math:`i` (:math:`k_i`).
#
# The score is then used to compute the sum of the incoming values,
# normalized over the weights of edges, stored in :math:`\textrm{wv}`.
# Then apply an affine layer to :math:`\textrm{wv}` to get the output
# :math:`o`:
#
# .. math::
#
#
# 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} \\
#
# 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 consists of
# multiple attention heads, in which each head refers to a single
# attention module. :math:`\textrm{wv}^{(i)}` for all the heads are
# concatenated and mapped to output :math:`o` with an affine layer:
#
# .. math::
#
#
# 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
# provides 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
# :math:`o` for post-processing.
#
# .. code::
#
# 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))
#
#
# How DGL implements Transformer with a graph neural network
# ----------------------------------------------------------
#
# You get 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
# ~~~~~~~~~~~~~~~
#
# 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 :math:`s_i` can attend to any other token :math:`s_j` (including
# self-loops). |image0|
# **Target language graph**. The graph is
# half-complete, in that :math:`t_i` attends only to :math:`t_j` if
# :math:`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 :math:`s_i` to every target token
# :math:`t_j`, meaning every target token can attend on source tokens.
# |image2|
#
# The full picture looks like this: |image3|
#
# Pre-build the graphs in dataset preparation stage.
#
# Message passing
# ~~~~~~~~~~~~~~~
#
# Once you define the graph structure, move on to defining the
# computation for message passing.
#
# Assuming that you have already computed all the queries :math:`q_i`, keys
# :math:`k_i` and values :math:`v_i`. For each node :math:`i` (no matter
# whether it is a source token or target token), you can decompose the
# attention computation into two steps:
#
# 1. **Message computation:** Compute attention score
# :math:`\mathrm{score}_{ij}` between :math:`i` and all nodes :math:`j`
# to be attended over, by taking the scaled-dot product between
# :math:`q_i` and :math:`k_j`. The message sent from :math:`j` to
# :math:`i` will consist of the score :math:`\mathrm{score}_{ij}` and
# the value :math:`v_j`.
# 2. **Message aggregation:** Aggregate the values :math:`v_j` from all
# :math:`j` according to the scores :math:`\mathrm{score}_{ij}`.
#
# Simple implementation
# ^^^^^^^^^^^^^^^^^^^^
#
# Message computation
# '''''''''''''''''''
#
# Compute ``score`` and send source node’s ``v`` to destination’s mailbox
#
# .. code::
#
# 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
#
# .. code::
#
# 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
# '''''''''''''''''''''''''
#
# .. code::
#
# 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, use DGL’s built-in
# functions, 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_e(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, you assemble those built-in functions into ``propagate_attention``,
# which is also the main graph operation function in the final
# implementation. To accelerate it, 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)))``
#
# .. math:: \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
# :math:`\textrm{wv}` is not normalized.
#
# - ``msg: fn.u_mul_e('v', 'score', 'v'), reduce: fn.sum('v', 'wv')``
#
# .. math:: \textrm{wv}_j=\sum_{i=1}^{N} \textrm{score}_{ij} \cdot v_i
#
# - ``msg: fn.copy_e('score', 'score'), reduce: fn.sum('score', 'z')``
#
# .. math:: \textrm{z}_j=\sum_{i=1}^{N} \textrm{score}_{ij}
#
# The normalization of :math:`\textrm{wv}` is left to post processing.
#
# .. code::
#
# 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.u_mul_e('v', 'score', 'v'), fn.copy_e('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:
#
# .. math::
#
#
# x \leftarrow \textrm{LayerNorm}(x) \\
# [q, k, v] \leftarrow [W_q, W_k, W_v ]\cdot x
#
# **Postprocessing** The postprocessing function ``post_funcs`` completes
# the whole computation correspond to one layer of the transformer: 1.
# Normalize :math:`\textrm{wv}` and get the output of Multi-Head Attention
# Layer :math:`o`.
#
# .. math::
#
#
# \textrm{wv} \leftarrow \frac{\textrm{wv}}{z} \\
# o \leftarrow W_o\cdot \textrm{wv} + b_o
#
# add residual connection:
#
# .. math::
#
#
# x \leftarrow x + o
#
# 2. Applying a two layer position-wise feed forward layer on :math:`x`
# then add residual connection:
#
# .. math::
#
#
# x \leftarrow x + \textrm{LayerNorm}(\textrm{FFN}(x))
#
# where :math:`\textrm{FFN}` refers to the feed forward function.
#
# .. code::
#
# 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, this 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|
#
# .. code:: python
#
# 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.u_mul_e('v', 'score', 'v'), fn.copy_e('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, you can create your 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 this
# implementation you don't use mask or padding, which makes the logic
# more clear and saves memory. The trade-off is that the implementation is
# slower.
#
# 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, 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. This tutorial focuses
# on the sequence to sequence learning: it’s a typical case to illustrate how it works.
#
# As for the dataset, there are two example 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::
# Training with wmt14 requires multi-GPU support and is not available. Contributions are welcome!
#
# Graph building
# ~~~~~~~~~~~~~~
#
# **Batching** This is similar to the way you handle Tree-LSTM. Build a graph pool in
# advance, including all possible combination of input lengths and output
# lengths. Then for each sample in a batch, call ``dgl.batch`` to batch
# graphs of their sizes together in to a single large graph.
#
# You can wrap the process of creating graph pool and building
# BatchedGraph in ``dataset.GraphPool`` and
# ``dataset.TranslationDataset``.
#
# .. code:: python
#
# 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:
#
# .. code::
#
# 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
# -------------------
#
# Train a one-head transformer with one layer, 128 dimension on copy
# task. Set other parameters to the default.
#
# Inference module is not included in this tutorial. It
# requires beam search. For a full implementation, see the `GitHub
# repo `__.
#
# .. code:: python
#
# from tqdm.auto 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, you can visualize the attention that the Transformer generates
# on copy task.
#
# .. code:: python
#
# 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 you 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 you see the diversity of different heads, which is what you would
# expect. 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 research 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**
# hereafter). This model is also known as the Adaptive Universal
# Transformer (AUT).
#
# In AUT, you maintain an active nodes list. In each step :math:`t`, we
# compute a halting probability: :math:`h (0`__.
#
# The figure below shows the effect of Adaptive Computational
# Time. Different positions of a sentence were revised different times.
#
# |image9|
#
# You can 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|
#
# .. |image0| image:: https://i.imgur.com/zV5LmTX.png
# .. |image1| image:: https://i.imgur.com/dETQMMx.png
# .. |image2| image:: https://i.imgur.com/hnGP229.png
# .. |image3| image:: https://i.imgur.com/Hj2rRGT.png
# .. |image4| image:: https://i.imgur.com/zlUpJ41.png
# .. |image5| image:: https://s1.ax1x.com/2018/12/06/F126xI.gif
# .. |image6| image:: https://i.imgur.com/HjYb7F2.png
# .. |image7| image:: https://i.imgur.com/383J5O5.png
# .. |image8| image:: https://i.imgur.com/c0UWB1V.png
# .. |image9| image:: https://s1.ax1x.com/2018/12/06/F1sGod.png
# .. |image10| image:: https://s1.ax1x.com/2018/12/06/F1r8Cq.gif
#
# .. note::
# The notebook itself is not executable due to many dependencies.
# Download `7_transformer.py `__,
# and copy the python script to directory ``examples/pytorch/transformer``
# then run ``python 7_transformer.py`` to see how it works.