pytorch 机器翻译 seq2seq 模型和注意力机制

序列到序列（seq2seq）的模型图示：

在很多任务场景中，我们需要处理变长序列，并输出另一个变长序列，如：机器翻译、文本摘要、语音识别等。

例：
input: 我是你爸爸 
output:  i m your father 

seq2seq 是处理这种问题的通用模型，如上图所示：用一个 RNN 将输入序列编码成一个 context （上下文）向量，然后从这个 context 向量解码出需要生成的序列，因而这个模型亦可成为 Encoder-Decoder （编码-解码）模型。

下面用机器翻译的模型来理解 seq2seq 的机理。

pytorch 实战

数据集

下载 pytorch_tutorial_data，其中文件 eng-fra.txt 内容如下，为 tab 间隔的英-法短句。

...
Tom passed away last year.	Tom est décédé l'année dernière.
Tom pulled the fire alarm.	Tom a tiré l'alarme d'incendie.
Tom raised his right hand.	Tom leva sa main droite.
Tom raised his right hand.	Tom a levé sa main droite.
Tom read the letter aloud.	Tom lut la lettre à voix haute.
Tom reads to his daughter.	Tom lit à sa fille.
Tom realized he was alone.	Tom réalisa qu'il était seul.
Tom really needs our help.	Tom a vraiment besoin de notre aide.
Tom refused to talk to me.	Tom a refusé de me parler.
Tom remembered everything.	Tom se souvenait de tout.
Tom rolled up his sleeves.	Tom retroussa ses manches.
Tom said he wasn't hungry.	Tom a dit qu'il n'avait pas faim.
Tom said he would be here.	Tom a dit qu'il serait ici.
Tom said that he liked me.	Tom a dit qu'il m'appréciait.
Tom said that he liked me.	Tom a dit qu'il m'aimait bien.
Tom said that he was busy.	Tom a dit qu'il était occupé.
Tom sat down next to Mary.	Tom s'assit à côté de Mary.
Tom sat down on the couch.	Tom s'assit sur le canapé.
Tom seemed very impressed.	Tom avait l'air très impressionné.
Tom seldom makes mistakes.	Tom fait rarement des erreurs.
Tom should have paid Mary.	Tom aurait dû payer Marie.
Tom should've been in bed.	Tom aurait dû être au lit.
...

from __future__ import unicode_literals, print_function, division
from io import open
import unicodedata
import string
import re
import random

import torch
import torch.nn as nn
from torch import optim
import torch.nn.functional as F

import matplotlib.pyplot as plt
%matplotlib inline
# plt.switch_backend('agg')
import matplotlib.ticker as ticker
import numpy as np

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

预处理

给每一种语言定义字典类 Lang，记录

• word2index 单词索引
• index2word 索引单词，其中包含两个特殊符号（Start Of Sentence, End Of Sentence），用于记录句子的起始和终止
• word2count 单词词频
• n_words 单词总数

SOS_token = 0
EOS_token = 1

class Lang:
    def __init__(self, name):
        self.name = name
        self.word2index = {}
        self.word2count = {}
        self.index2word = {0: "SOS", 1: "EOS"}
        self.n_words = 2  # Count SOS and EOS

    def addSentence(self, sentence):
        for word in sentence.split(' '):
            self.addWord(word)

    def addWord(self, word):
        if word not in self.word2index:
            self.word2index[word] = self.n_words
            self.word2count[word] = 1
            self.index2word[self.n_words] = word
            self.n_words += 1
        else:
            self.word2count[word] += 1

将 unicode 转换成 ASCII码，降低字符的表示维度，因为法语中有些字符可能带帽子，不在26个字母内，如: Ślusàrski

# Turn a Unicode string to plain ASCII, thanks to
# https://stackoverflow.com/a/518232/2809427
def unicodeToAscii(s):
    return ''.join(
        c for c in unicodedata.normalize('NFD', s)
        if unicodedata.category(c) != 'Mn'
    )

# Lowercase, trim, and remove non-letter characters

def normalizeString(s):
    s = unicodeToAscii(s.lower().strip())
    s = re.sub(r"([.!?])", r" \1", s)
    s = re.sub(r"[^a-zA-Z.!?]+", r" ", s)
    return s

读取数据文件到列表中

def readLangs(lang1, lang2, reverse=False):
    print("Reading lines...")

    # Read the file and split into lines
    lines = open('data/data/%s-%s.txt' % (lang1, lang2), encoding='utf-8').\
        read().strip().split('\n')

    # Split every line into pairs and normalize
    pairs = [[normalizeString(s) for s in l.split('\t')] for l in lines]

    # Reverse pairs, make Lang instances
    if reverse:
        pairs = [list(reversed(p)) for p in pairs]
        input_lang = Lang(lang2)
        output_lang = Lang(lang1)
    else:
        input_lang = Lang(lang1)
        output_lang = Lang(lang2)

    return input_lang, output_lang, pairs

对语料库中的句子过滤一遍，只用以如下字符开头语句训练模型

MAX_LENGTH = 10

eng_prefixes = (
    "i am ", "i m ",
    "he is", "he s ",
    "she is", "she s ",
    "you are", "you re ",
    "we are", "we re ",
    "they are", "they re "
)


def filterPair(p):
    return len(p[0].split(' ')) < MAX_LENGTH and \
        len(p[1].split(' ')) < MAX_LENGTH and \
        p[1].startswith(eng_prefixes)


def filterPairs(pairs):
    return [pair for pair in pairs if filterPair(pair)]

def prepareData(lang1, lang2, reverse=False):
    input_lang, output_lang, pairs = readLangs(lang1, lang2, reverse)
    print("Read %s sentence pairs" % len(pairs))
    pairs = filterPairs(pairs)
    print("Trimmed to %s sentence pairs" % len(pairs))
    print("Counting words...")
    for pair in pairs:
        input_lang.addSentence(pair[0])
        output_lang.addSentence(pair[1])
    print("Counted words:")
    print(input_lang.name, input_lang.n_words)
    print(output_lang.name, output_lang.n_words)
    return input_lang, output_lang, pairs

input_lang, output_lang, pairs = prepareData('eng', 'fra', True)
print(random.choice(pairs))

'''
Reading lines...
Read 135842 sentence pairs
Trimmed to 10599 sentence pairs
Counting words...
Counted words:
fra 4345
eng 2803
['je ne le fais plus .', 'i m not doing it anymore .']
'''

将字符串转换成张量

def indexesFromSentence(lang, sentence):
    return [lang.word2index[word] for word in sentence.split(' ')]


def tensorFromSentence(lang, sentence):
    indexes = indexesFromSentence(lang, sentence)
    indexes.append(EOS_token)
    return torch.tensor(indexes, dtype=torch.long, device=device).view(-1, 1)


def tensorsFromPair(pair):
    input_tensor = tensorFromSentence(input_lang, pair[0])
    target_tensor = tensorFromSentence(output_lang, pair[1])
    return (input_tensor, target_tensor)

记录时间

import time
import math

def asMinutes(s):
    m = math.floor(s / 60)
    s -= m * 60
    return '%dm %ds' % (m, s)

def timeSince(since, percent):
    now = time.time()
    s = now - since
    es = s / (percent)
    rs = es - s
    return '%s (- %s)' % (asMinutes(s), asMinutes(rs))

定义 编码器

class EncoderRNN(nn.Module):
    def __init__(self, input_size, hidden_size):
        super(EncoderRNN, self).__init__()
        self.hidden_size = hidden_size

        self.embedding = nn.Embedding(input_size, hidden_size)
        self.gru = nn.GRU(hidden_size, hidden_size)

    def forward(self, input, hidden):
        embedded = self.embedding(input).view(1, 1, -1)
        output = embedded
        output, hidden = self.gru(output, hidden)
        return output, hidden

    def initHidden(self):
        return torch.zeros(1, 1, self.hidden_size, device=device)

定义 解码器

class AttnDecoderRNN(nn.Module):
    def __init__(self, hidden_size, output_size, dropout_p=0.1, max_length=MAX_LENGTH):
        super(AttnDecoderRNN, self).__init__()
        self.hidden_size = hidden_size
        self.output_size = output_size
        self.dropout_p = dropout_p
        self.max_length = max_length

        self.embedding = nn.Embedding(self.output_size, self.hidden_size)
        self.attn = nn.Linear(self.hidden_size * 2, self.max_length)
        self.attn_combine = nn.Linear(self.hidden_size * 2, self.hidden_size)
        self.dropout = nn.Dropout(self.dropout_p)
        self.gru = nn.GRU(self.hidden_size, self.hidden_size)
        self.out = nn.Linear(self.hidden_size, self.output_size)

    def forward(self, input, hidden, encoder_outputs):
        embedded = self.embedding(input).view(1, 1, -1)
        embedded = self.dropout(embedded)

        attn_weights = F.softmax(
            self.attn(torch.cat((embedded[0], hidden[0]), 1)), dim=1)
        attn_applied = torch.bmm(attn_weights.unsqueeze(0),
                                 encoder_outputs.unsqueeze(0))

        output = torch.cat((embedded[0], attn_applied[0]), 1)
        output = self.attn_combine(output).unsqueeze(0)

        output = F.relu(output)
        output, hidden = self.gru(output, hidden)

        output = F.log_softmax(self.out(output[0]), dim=1)
        return output, hidden, attn_weights

    def initHidden(self):
        return torch.zeros(1, 1, self.hidden_size, device=device)

这个解码器中加入了注意力机制。

没有加入注意力前，解码器在每一次迭代时都将整个 context 向量作为输入，以下面这个翻译为例：

例：
input: 我是你爸爸 
output:  i m your father 

编码器将 “我是你爸爸 ” 编码成向量 context，context 这个向量就包含了这句话的全部语义信息。没有注意力机制的解码器不论是翻译到 ‘m’，还是 ‘your’，还是 ‘father’ 的时候，都将整个 context 作为那一步的输入。

更合理的做法是：翻译出 ‘m’ 时，应该参考 ‘是’ 这个字；翻译出 ‘your’ 时，应该参考 ‘你’ 这个字。

也就是说，解码器的每一步，只需要参考 context 的部分信息，这就是注意力机制。从上面的网络图来看，就是用一个掩码 attn_weights 将编码器的输出 encoder_outputs（context） 过滤一次，得到的 attn_applied 就是在 context 里在当前迭代步值得注意的片段，将其作为当前步的输入信息。

训练

训练单条语句

teacher_forcing_ratio = 0.5


def train(input_tensor, target_tensor, encoder, decoder, encoder_optimizer, decoder_optimizer, criterion, max_length=MAX_LENGTH):
    encoder_hidden = encoder.initHidden()

    encoder_optimizer.zero_grad()
    decoder_optimizer.zero_grad()

    input_length = input_tensor.size(0)
    target_length = target_tensor.size(0)

    encoder_outputs = torch.zeros(max_length, encoder.hidden_size, device=device)

    loss = 0

    for ei in range(input_length):
        encoder_output, encoder_hidden = encoder(
            input_tensor[ei], encoder_hidden)
        encoder_outputs[ei] = encoder_output[0, 0]

    decoder_input = torch.tensor([[SOS_token]], device=device)

    decoder_hidden = encoder_hidden

    use_teacher_forcing = True if random.random() < teacher_forcing_ratio else False

    if use_teacher_forcing:
        # Teacher forcing: Feed the target as the next input
        for di in range(target_length):
            decoder_output, decoder_hidden, decoder_attention = decoder(
                decoder_input, decoder_hidden, encoder_outputs)
            loss += criterion(decoder_output, target_tensor[di])
            decoder_input = target_tensor[di]  # Teacher forcing

    else:
        # Without teacher forcing: use its own predictions as the next input
        for di in range(target_length):
            decoder_output, decoder_hidden, decoder_attention = decoder(
                decoder_input, decoder_hidden, encoder_outputs)
            topv, topi = decoder_output.topk(1)
            decoder_input = topi.squeeze().detach()  # detach from history as input

            loss += criterion(decoder_output, target_tensor[di])
            if decoder_input.item() == EOS_token:
                break

    loss.backward()

    encoder_optimizer.step()
    decoder_optimizer.step()

    return loss.item() / target_length

训练批量语句

def trainIters(encoder, decoder, n_iters, print_every=1000, plot_every=100, learning_rate=0.01):
    start = time.time()
    plot_losses = []
    print_loss_total = 0  # Reset every print_every
    plot_loss_total = 0  # Reset every plot_every

    encoder_optimizer = optim.SGD(encoder.parameters(), lr=learning_rate)
    decoder_optimizer = optim.SGD(decoder.parameters(), lr=learning_rate)
    training_pairs = [tensorsFromPair(random.choice(pairs))
                      for i in range(n_iters)]
    criterion = nn.NLLLoss()

    for iter in range(1, n_iters + 1):
        training_pair = training_pairs[iter - 1]
        input_tensor = training_pair[0]
        target_tensor = training_pair[1]

        loss = train(input_tensor, target_tensor, encoder,
                     decoder, encoder_optimizer, decoder_optimizer, criterion)
        print_loss_total += loss
        plot_loss_total += loss

        if iter % print_every == 0:
            print_loss_avg = print_loss_total / print_every
            print_loss_total = 0
            print('%s (%d %d%%) %.4f' % (timeSince(start, iter / n_iters),
                                         iter, iter / n_iters * 100, print_loss_avg))

        if iter % plot_every == 0:
            plot_loss_avg = plot_loss_total / plot_every
            plot_losses.append(plot_loss_avg)
            plot_loss_total = 0

    showPlot(plot_losses)

def showPlot(points):
    plt.figure()
    fig, ax = plt.subplots()
    # this locator puts ticks at regular intervals
    loc = ticker.MultipleLocator(base=0.2)
    ax.yaxis.set_major_locator(loc)
    plt.plot(points)

训练

hidden_size = 256
encoder = EncoderRNN(input_lang.n_words, hidden_size).to(device)
attn_decoder1 = AttnDecoderRNN(hidden_size, output_lang.n_words, dropout_p=0.1).to(device)

trainIters(encoder, attn_decoder1, 75000, print_every=5000)
'''
2m 55s (- 40m 57s) (5000 6%) 1.7566
5m 49s (- 37m 52s) (10000 13%) 1.2538
8m 41s (- 34m 46s) (15000 20%) 1.0629
11m 37s (- 31m 59s) (20000 26%) 0.9481
14m 30s (- 29m 0s) (25000 33%) 0.8491
17m 22s (- 26m 3s) (30000 40%) 0.7600
20m 14s (- 23m 7s) (35000 46%) 0.6800
23m 7s (- 20m 14s) (40000 53%) 0.6074
26m 1s (- 17m 20s) (45000 60%) 0.5736
28m 54s (- 14m 27s) (50000 66%) 0.5043
31m 48s (- 11m 34s) (55000 73%) 0.4845
34m 42s (- 8m 40s) (60000 80%) 0.4332
37m 34s (- 5m 46s) (65000 86%) 0.4044
40m 28s (- 2m 53s) (70000 93%) 0.3875
43m 23s (- 0m 0s) (75000 100%) 0.3679
'''

测试结果

测评单条语句

def evaluate(encoder, decoder, sentence, max_length=MAX_LENGTH):
    with torch.no_grad():
        input_tensor = tensorFromSentence(input_lang, sentence)
        input_length = input_tensor.size()[0]
        encoder_hidden = encoder.initHidden()

        encoder_outputs = torch.zeros(max_length, encoder.hidden_size, device=device)

        for ei in range(input_length):
            encoder_output, encoder_hidden = encoder(input_tensor[ei],
                                                     encoder_hidden)
            encoder_outputs[ei] += encoder_output[0, 0]

        decoder_input = torch.tensor([[SOS_token]], device=device)  # SOS

        decoder_hidden = encoder_hidden

        decoded_words = []
        decoder_attentions = torch.zeros(max_length, max_length)

        for di in range(max_length):
            decoder_output, decoder_hidden, decoder_attention = decoder(
                decoder_input, decoder_hidden, encoder_outputs)
            decoder_attentions[di] = decoder_attention.data
            topv, topi = decoder_output.data.topk(1)
            if topi.item() == EOS_token:
                decoded_words.append('')
                break
            else:
                decoded_words.append(output_lang.index2word[topi.item()])

            decoder_input = topi.squeeze().detach()

        return decoded_words, decoder_attentions[:di + 1]

测评多条语句

def evaluateRandomly(encoder, decoder, n=10):
    for i in range(n):
        pair = random.choice(pairs)
        print('>', pair[0])
        print('=', pair[1])
        output_words, attentions = evaluate(encoder, decoder, pair[0])
        output_sentence = ' '.join(output_words)
        print('<', output_sentence)
        print('')

evaluateRandomly(encoder1, attn_decoder1)
'''
> ils ne sont pas satisfaits .
= they re not happy .
< they re unhappy . 

> nous ne sommes pas ici pour t arreter .
= we are not here to arrest you .
< we are not here to ask you . 

> je ne peux pas reparer l ordinateur .
= i m not able to fix the computer .
< i m not able to fix the . 

> je suis tres heureux maintenant .
= i m very happy now .
< i m very happy now . 

> elle n est pas poete mais romanciere .
= she is not a poet but a novelist .
< she is not a but but a 

> elle est sa veritable mere .
= she is his real mother .
< she is her mother mother . 

> je suppose que c est votre pere .
= i m assuming this is your father .
< i m assuming this is your father . 

> tu es un etre mauvais .
= you are a bad person .
< you are a bad person . 

> tu es surmene .
= you re overworked .
< you are overworked . 

> je viens de l exterieur de la ville .
= i m from out of town .
< i m from out of town . 

'''

注意力可视化

def showAttention(input_sentence, output_words, attentions):
    # Set up figure with colorbar
    fig = plt.figure()
    ax = fig.add_subplot(111)
    cax = ax.matshow(attentions.numpy(), cmap='bone')
    fig.colorbar(cax)

    # Set up axes
    ax.set_xticklabels([''] + input_sentence.split(' ') +
                       [''], rotation=90)
    ax.set_yticklabels([''] + output_words)

    # Show label at every tick
    ax.xaxis.set_major_locator(ticker.MultipleLocator(1))
    ax.yaxis.set_major_locator(ticker.MultipleLocator(1))

    plt.show()


def evaluateAndShowAttention(input_sentence):
    output_words, attentions = evaluate(
        encoder1, attn_decoder1, input_sentence)
    print('input =', input_sentence)
    print('output =', ' '.join(output_words))
    showAttention(input_sentence, output_words, attentions)


evaluateAndShowAttention("elle a cinq ans de moins que moi .")

evaluateAndShowAttention("elle est trop petit .")

evaluateAndShowAttention("je ne crains pas de mourir .")

evaluateAndShowAttention("c est un jeune directeur plein de talent .")

input = elle a cinq ans de moins que moi .
output = she is two years younger than me .

input = elle est trop petit .
output = she s too drunk .

input = je ne crains pas de mourir .
output = i m not scared to die .

input = c est un jeune directeur plein de talent .
output = he is a talented young talented .