使用TorchText进行语言翻译

本教程展示了如何使用torchtext来预处理来自一个包含英语和德语句子的知名数据集的数据，并使用它来训练一个带有注意力的序列到序列模型，该模型可以将德语句子翻译成英语。

它基于 这个教程 来自PyTorch社区成员Ben Trevett 在Ben的许可下。我们通过删除一些遗留代码来更新教程。

在本教程结束时，您将能够将句子预处理为用于NLP建模的张量，并使用torch.utils.data.DataLoader来训练和验证模型。

数据处理

torchtext 提供了创建数据集的工具，这些数据集可以轻松地进行迭代，以用于创建语言翻译模型。在本示例中，我们将展示如何对原始文本句子进行分词、构建词汇表，并将分词结果转换为张量。

注意：本教程中的分词需要Spacy。我们使用Spacy是因为它对非英语语言的分词提供了强大的支持。torchtext提供了一个basic_english分词器，并支持其他英语分词器（例如Moses），但对于需要多种语言的翻译任务，Spacy是最佳选择。

要运行本教程，首先使用pip或conda安装spacy。 接下来，下载用于英语和德语Spacy分词器的原始数据：

python -m spacy download en
python -m spacy download de

import torchtext
import torch
from torchtext.data.utils import get_tokenizer
from collections import Counter
from torchtext.vocab import Vocab
from torchtext.utils import download_from_url, extract_archive
import io

url_base = 'https://raw.githubusercontent.com/multi30k/dataset/master/data/task1/raw/'
train_urls = ('train.de.gz', 'train.en.gz')
val_urls = ('val.de.gz', 'val.en.gz')
test_urls = ('test_2016_flickr.de.gz', 'test_2016_flickr.en.gz')

train_filepaths = [extract_archive(download_from_url(url_base + url))[0] for url in train_urls]
val_filepaths = [extract_archive(download_from_url(url_base + url))[0] for url in val_urls]
test_filepaths = [extract_archive(download_from_url(url_base + url))[0] for url in test_urls]

de_tokenizer = get_tokenizer('spacy', language='de')
en_tokenizer = get_tokenizer('spacy', language='en')

def build_vocab(filepath, tokenizer):
  counter = Counter()
  with io.open(filepath, encoding="utf8") as f:
    for string_ in f:
      counter.update(tokenizer(string_))
  return Vocab(counter, specials=['<unk>', '<pad>', '<bos>', '<eos>'])

de_vocab = build_vocab(train_filepaths[0], de_tokenizer)
en_vocab = build_vocab(train_filepaths[1], en_tokenizer)

def data_process(filepaths):
  raw_de_iter = iter(io.open(filepaths[0], encoding="utf8"))
  raw_en_iter = iter(io.open(filepaths[1], encoding="utf8"))
  data = []
  for (raw_de, raw_en) in zip(raw_de_iter, raw_en_iter):
    de_tensor_ = torch.tensor([de_vocab[token] for token in de_tokenizer(raw_de)],
                            dtype=torch.long)
    en_tensor_ = torch.tensor([en_vocab[token] for token in en_tokenizer(raw_en)],
                            dtype=torch.long)
    data.append((de_tensor_, en_tensor_))
  return data

train_data = data_process(train_filepaths)
val_data = data_process(val_filepaths)
test_data = data_process(test_filepaths)

DataLoader

我们将使用的最后一个torch特定功能是DataLoader，它易于使用，因为它将数据作为其第一个参数。具体来说，正如文档所说：DataLoader结合了一个数据集和一个采样器，并提供了对给定数据集的可迭代访问。DataLoader支持单进程或多进程加载的映射式和可迭代式数据集，自定义加载顺序以及可选的自动批处理（整理）和内存固定。

请注意collate_fn（可选），它合并样本列表以形成一个小批量的张量。在使用从映射样式数据集进行批量加载时使用。

import torch

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

BATCH_SIZE = 128
PAD_IDX = de_vocab['<pad>']
BOS_IDX = de_vocab['<bos>']
EOS_IDX = de_vocab['<eos>']

from torch.nn.utils.rnn import pad_sequence
from torch.utils.data import DataLoader

def generate_batch(data_batch):
  de_batch, en_batch = [], []
  for (de_item, en_item) in data_batch:
    de_batch.append(torch.cat([torch.tensor([BOS_IDX]), de_item, torch.tensor([EOS_IDX])], dim=0))
    en_batch.append(torch.cat([torch.tensor([BOS_IDX]), en_item, torch.tensor([EOS_IDX])], dim=0))
  de_batch = pad_sequence(de_batch, padding_value=PAD_IDX)
  en_batch = pad_sequence(en_batch, padding_value=PAD_IDX)
  return de_batch, en_batch

train_iter = DataLoader(train_data, batch_size=BATCH_SIZE,
                        shuffle=True, collate_fn=generate_batch)
valid_iter = DataLoader(val_data, batch_size=BATCH_SIZE,
                        shuffle=True, collate_fn=generate_batch)
test_iter = DataLoader(test_data, batch_size=BATCH_SIZE,
                       shuffle=True, collate_fn=generate_batch)

定义我们的nn.Module和Optimizer

这基本上就是从torchtext的角度来看的内容：数据集构建完成后，迭代器定义好了，本教程的其余部分只是将我们的模型定义为nn.Module，以及一个Optimizer，然后进行训练。

我们的模型具体遵循了这里描述的架构（你可以找到一个注释更多的版本这里）。

注意：这个模型只是一个可以用于语言翻译的示例模型；我们选择它是因为它是该任务的标准模型，而不是因为它是最推荐的翻译模型。正如你可能知道的，目前最先进的模型是基于Transformers的；你可以在这里查看PyTorch实现Transformer层的能力here；特别是，下面模型中使用的“注意力”与Transformer模型中的多头自注意力不同。

import random
from typing import Tuple

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


class Encoder(nn.Module):
    def __init__(self,
                 input_dim: int,
                 emb_dim: int,
                 enc_hid_dim: int,
                 dec_hid_dim: int,
                 dropout: float):
        super().__init__()

        self.input_dim = input_dim
        self.emb_dim = emb_dim
        self.enc_hid_dim = enc_hid_dim
        self.dec_hid_dim = dec_hid_dim
        self.dropout = dropout

        self.embedding = nn.Embedding(input_dim, emb_dim)

        self.rnn = nn.GRU(emb_dim, enc_hid_dim, bidirectional = True)

        self.fc = nn.Linear(enc_hid_dim * 2, dec_hid_dim)

        self.dropout = nn.Dropout(dropout)

    def forward(self,
                src: Tensor) -> Tuple[Tensor]:

        embedded = self.dropout(self.embedding(src))

        outputs, hidden = self.rnn(embedded)

        hidden = torch.tanh(self.fc(torch.cat((hidden[-2,:,:], hidden[-1,:,:]), dim = 1)))

        return outputs, hidden


class Attention(nn.Module):
    def __init__(self,
                 enc_hid_dim: int,
                 dec_hid_dim: int,
                 attn_dim: int):
        super().__init__()

        self.enc_hid_dim = enc_hid_dim
        self.dec_hid_dim = dec_hid_dim

        self.attn_in = (enc_hid_dim * 2) + dec_hid_dim

        self.attn = nn.Linear(self.attn_in, attn_dim)

    def forward(self,
                decoder_hidden: Tensor,
                encoder_outputs: Tensor) -> Tensor:

        src_len = encoder_outputs.shape[0]

        repeated_decoder_hidden = decoder_hidden.unsqueeze(1).repeat(1, src_len, 1)

        encoder_outputs = encoder_outputs.permute(1, 0, 2)

        energy = torch.tanh(self.attn(torch.cat((
            repeated_decoder_hidden,
            encoder_outputs),
            dim = 2)))

        attention = torch.sum(energy, dim=2)

        return F.softmax(attention, dim=1)


class Decoder(nn.Module):
    def __init__(self,
                 output_dim: int,
                 emb_dim: int,
                 enc_hid_dim: int,
                 dec_hid_dim: int,
                 dropout: int,
                 attention: nn.Module):
        super().__init__()

        self.emb_dim = emb_dim
        self.enc_hid_dim = enc_hid_dim
        self.dec_hid_dim = dec_hid_dim
        self.output_dim = output_dim
        self.dropout = dropout
        self.attention = attention

        self.embedding = nn.Embedding(output_dim, emb_dim)

        self.rnn = nn.GRU((enc_hid_dim * 2) + emb_dim, dec_hid_dim)

        self.out = nn.Linear(self.attention.attn_in + emb_dim, output_dim)

        self.dropout = nn.Dropout(dropout)


    def _weighted_encoder_rep(self,
                              decoder_hidden: Tensor,
                              encoder_outputs: Tensor) -> Tensor:

        a = self.attention(decoder_hidden, encoder_outputs)

        a = a.unsqueeze(1)

        encoder_outputs = encoder_outputs.permute(1, 0, 2)

        weighted_encoder_rep = torch.bmm(a, encoder_outputs)

        weighted_encoder_rep = weighted_encoder_rep.permute(1, 0, 2)

        return weighted_encoder_rep


    def forward(self,
                input: Tensor,
                decoder_hidden: Tensor,
                encoder_outputs: Tensor) -> Tuple[Tensor]:

        input = input.unsqueeze(0)

        embedded = self.dropout(self.embedding(input))

        weighted_encoder_rep = self._weighted_encoder_rep(decoder_hidden,
                                                          encoder_outputs)

        rnn_input = torch.cat((embedded, weighted_encoder_rep), dim = 2)

        output, decoder_hidden = self.rnn(rnn_input, decoder_hidden.unsqueeze(0))

        embedded = embedded.squeeze(0)
        output = output.squeeze(0)
        weighted_encoder_rep = weighted_encoder_rep.squeeze(0)

        output = self.out(torch.cat((output,
                                     weighted_encoder_rep,
                                     embedded), dim = 1))

        return output, decoder_hidden.squeeze(0)


class Seq2Seq(nn.Module):
    def __init__(self,
                 encoder: nn.Module,
                 decoder: nn.Module,
                 device: torch.device):
        super().__init__()

        self.encoder = encoder
        self.decoder = decoder
        self.device = device

    def forward(self,
                src: Tensor,
                trg: Tensor,
                teacher_forcing_ratio: float = 0.5) -> Tensor:

        batch_size = src.shape[1]
        max_len = trg.shape[0]
        trg_vocab_size = self.decoder.output_dim

        outputs = torch.zeros(max_len, batch_size, trg_vocab_size).to(self.device)

        encoder_outputs, hidden = self.encoder(src)

        # first input to the decoder is the <sos> token
        output = trg[0,:]

        for t in range(1, max_len):
            output, hidden = self.decoder(output, hidden, encoder_outputs)
            outputs[t] = output
            teacher_force = random.random() < teacher_forcing_ratio
            top1 = output.max(1)[1]
            output = (trg[t] if teacher_force else top1)

        return outputs


INPUT_DIM = len(de_vocab)
OUTPUT_DIM = len(en_vocab)
# ENC_EMB_DIM = 256
# DEC_EMB_DIM = 256
# ENC_HID_DIM = 512
# DEC_HID_DIM = 512
# ATTN_DIM = 64
# ENC_DROPOUT = 0.5
# DEC_DROPOUT = 0.5

ENC_EMB_DIM = 32
DEC_EMB_DIM = 32
ENC_HID_DIM = 64
DEC_HID_DIM = 64
ATTN_DIM = 8
ENC_DROPOUT = 0.5
DEC_DROPOUT = 0.5

enc = Encoder(INPUT_DIM, ENC_EMB_DIM, ENC_HID_DIM, DEC_HID_DIM, ENC_DROPOUT)

attn = Attention(ENC_HID_DIM, DEC_HID_DIM, ATTN_DIM)

dec = Decoder(OUTPUT_DIM, DEC_EMB_DIM, ENC_HID_DIM, DEC_HID_DIM, DEC_DROPOUT, attn)

model = Seq2Seq(enc, dec, device).to(device)


def init_weights(m: nn.Module):
    for name, param in m.named_parameters():
        if 'weight' in name:
            nn.init.normal_(param.data, mean=0, std=0.01)
        else:
            nn.init.constant_(param.data, 0)


model.apply(init_weights)

optimizer = optim.Adam(model.parameters())


def count_parameters(model: nn.Module):
    return sum(p.numel() for p in model.parameters() if p.requires_grad)


print(f'The model has {count_parameters(model):,} trainable parameters')

注意：在评估语言翻译模型的性能时，我们必须告诉nn.CrossEntropyLoss函数忽略目标仅为填充的索引。

PAD_IDX = en_vocab.stoi['<pad>']

criterion = nn.CrossEntropyLoss(ignore_index=PAD_IDX)

最后，我们可以训练并评估这个模型：

import math
import time


def train(model: nn.Module,
          iterator: torch.utils.data.DataLoader,
          optimizer: optim.Optimizer,
          criterion: nn.Module,
          clip: float):

    model.train()

    epoch_loss = 0

    for _, (src, trg) in enumerate(iterator):
        src, trg = src.to(device), trg.to(device)

        optimizer.zero_grad()

        output = model(src, trg)

        output = output[1:].view(-1, output.shape[-1])
        trg = trg[1:].view(-1)

        loss = criterion(output, trg)

        loss.backward()

        torch.nn.utils.clip_grad_norm_(model.parameters(), clip)

        optimizer.step()

        epoch_loss += loss.item()

    return epoch_loss / len(iterator)


def evaluate(model: nn.Module,
             iterator: torch.utils.data.DataLoader,
             criterion: nn.Module):

    model.eval()

    epoch_loss = 0

    with torch.no_grad():

        for _, (src, trg) in enumerate(iterator):
            src, trg = src.to(device), trg.to(device)

            output = model(src, trg, 0) #turn off teacher forcing

            output = output[1:].view(-1, output.shape[-1])
            trg = trg[1:].view(-1)

            loss = criterion(output, trg)

            epoch_loss += loss.item()

    return epoch_loss / len(iterator)


def epoch_time(start_time: int,
               end_time: int):
    elapsed_time = end_time - start_time
    elapsed_mins = int(elapsed_time / 60)
    elapsed_secs = int(elapsed_time - (elapsed_mins * 60))
    return elapsed_mins, elapsed_secs


N_EPOCHS = 10
CLIP = 1

best_valid_loss = float('inf')

for epoch in range(N_EPOCHS):

    start_time = time.time()

    train_loss = train(model, train_iter, optimizer, criterion, CLIP)
    valid_loss = evaluate(model, valid_iter, criterion)

    end_time = time.time()

    epoch_mins, epoch_secs = epoch_time(start_time, end_time)

    print(f'Epoch: {epoch+1:02} | Time: {epoch_mins}m {epoch_secs}s')
    print(f'\tTrain Loss: {train_loss:.3f} | Train PPL: {math.exp(train_loss):7.3f}')
    print(f'\t Val. Loss: {valid_loss:.3f} |  Val. PPL: {math.exp(valid_loss):7.3f}')

test_loss = evaluate(model, test_iter, criterion)

print(f'| Test Loss: {test_loss:.3f} | Test PPL: {math.exp(test_loss):7.3f} |')

下一步

• Check out the rest of Ben Trevett’s tutorials using torchtext here
• Stay tuned for a tutorial using other torchtext features along with nn.Transformer for language modeling via next word prediction!