NLP扎实基础2：Word2vec模型CBOW Pytorch复现

Word2vec模型简介请参考：NLP扎实基础1：Word2vec模型Skip-Gram Pytorch复现

CBOW模型可以参考论文：

Mikolov, Tomas, et al. “Efficient estimation of word representations in vector space.” arXiv preprint arXiv:1301.3781 (2013).

CBOW算法流程

CBOW的流程示例如下：

步骤如下：

1. 首先使用滑动窗口依次滑动整个句子
2. 将窗口中间的词拿出来，使用周围的词来预测中间的词
3. 将预测任务当做是分类任务，预测的目标是所有的词
4. 计算预测结果与实际结果的交叉熵，作为loss

举个例子：

• 窗口=4（左边取2个，右边取2个）
• batch=27（一次训练27个例子）
• 词表中一共有300w个单词
• 每个词用300维来表示

使用Cbow训练时，整个的流程如下：

Pytorch 复现

import numpy as np
from torchtext.vocab import vocab
from collections import Counter, OrderedDict
from torch.utils.data import Dataset, DataLoader
from torchtext.transforms import VocabTransform  # 注意：torchtext版本0.12+
import torch
from torch import nn
from torch.nn import functional as F


def get_text():
    sentence_list = [  # 假设这是全部的训练语料
        "nlp drives computer programs that translate text from one language to another",
        "nlp combines computational linguistics rule based modeling of human language with statistical",
        "nlp model respond to text or voice data and respond with text",
    ]
    return sentence_list


class CbowDataSet(Dataset):
    def __init__(self, text_list, side_window=3):
        """
        构造Word2vec的CBOW采样Dataset
        :param text_list: 语料
        :param side_window: 单侧正例（构造背景词）采样数，总正例是：2 * side_window
        """
        super(CbowDataSet, self).__init__()
        self.side_window = side_window
        text_vocab, vocab_transform = self.reform_vocab(text_list)
        self.text_list = text_list  # 原始文本
        self.text_vocab = text_vocab  # torchtext的vocab
        self.vocab_transform = vocab_transform  # torchtext的vocab_transform
        self.cbow_data = self.generate_cbow()

    def __len__(self):
        return len(self.cbow_data)

    def __getitem__(self, idx):
        data_row = self.cbow_data[idx]
        return data_row[0], data_row[1]

    def reform_vocab(self, text_list):
        """根据语料构造torchtext的vocab"""
        total_word_list = []
        for _ in text_list:  # 将嵌套的列表([[xx,xx],[xx,xx]...])拉平 ([xx,xx,xx...])
            total_word_list += _.split(" ")
        counter = Counter(total_word_list)  # 统计计数
        sorted_by_freq_tuples = sorted(counter.items(), key=lambda x: x[1], reverse=True)  # 构造成可接受的格式：[(单词,num), ...]
        ordered_dict = OrderedDict(sorted_by_freq_tuples)
        # 开始构造 vocab
        special_token = ["", ""]  # 特殊字符
        text_vocab = vocab(ordered_dict, specials=special_token)  # 单词转token，specials里是特殊字符，可以为空
        text_vocab.set_default_index(0)
        vocab_transform = VocabTransform(text_vocab)
        return text_vocab, vocab_transform

    def generate_cbow(self):
        """生成CBOW的训练数据"""
        cbow_data = []
        for sentence in self.text_list:
            sentence_id_list = np.array(self.vocab_transform(sentence.split(' ')))
            for center_index in range(
                    self.side_window, len(sentence_id_list) - self.side_window):  # 防止前面或后面取不到足够的值，这是取index的上下界
                pos_index = list(range(center_index - self.side_window, center_index + self.side_window + 1))
                del pos_index[self.side_window]
                cbow_data.append([sentence_id_list[center_index], sentence_id_list[pos_index]])
        return cbow_data

    def get_vocab_transform(self):
        return self.vocab_transform

    def get_vocab_size(self):
        return len(self.text_vocab)


class Word2VecModel(nn.Module):
    def __init__(self, vocab_size, batch_size, word_embedding_size=100, hidden=64):
        """
        Word2vec模型CBOW实现
        :param vocab_size: 单词个数
        :param word_embedding_size: 每个词的词向量维度
        :param hidden: 隐层维度
        """
        super(Word2VecModel, self).__init__()
        self.vocab_size = vocab_size
        self.word_embedding_size = word_embedding_size
        self.hidden = hidden
        self.batch_size = batch_size
        self.word_embedding = nn.Embedding(self.vocab_size, self.word_embedding_size)  # token对应的embedding
        # 建模
        self.linear_in = nn.Linear(self.word_embedding_size, self.hidden)
        self.linear_out = nn.Linear(self.hidden, self.vocab_size)

    def forward(self, input_labels):
        around_embedding = self.word_embedding(input_labels)
        avg_around_embedding = torch.mean(around_embedding, dim=1)  # 1. 输入的词向量对应位置求平均
        in_emb = F.relu(self.linear_in(avg_around_embedding))  # 2. 过第一个linear，使用relu激活函数
        out_emb = F.log_softmax(self.linear_out(in_emb))  # 3. 过第二个linear，得到维度是：[batch_size, 单词总数]
        return out_emb

    def get_embedding(self, token_list: list):
        return self.word_embedding(torch.Tensor(token_list).long())


def main():
    batch_size = 7
    sentence_list = get_text()
    cbow_data_set = CbowDataSet(sentence_list)  # 构造 DataSet
    data_loader = DataLoader(cbow_data_set, batch_size=batch_size, drop_last=True)  # 将DataSet封装成DataLoader
    # 开始训练
    model = Word2VecModel(cbow_data_set.get_vocab_size(), batch_size)
    optimizer = torch.optim.Adam(model.parameters())
    criterion = nn.CrossEntropyLoss()
    for _epoch_i in range(100):
        loss_list = []
        for center_token, back_token in data_loader:
            # 开始训练
            optimizer.zero_grad()
            model_out = model(back_token)
            loss = criterion(model_out, center_token)
            loss.backward()
            optimizer.step()
            loss_list.append(loss.item())
        print("训练中：", _epoch_i, "Loss:", np.sum(loss_list))

    # 最后测试一下
    # 得到: nlp can translate text from one language to another 的词向量
    sentence = "nlp can translate text from one language to another"
    vocab_transform = cbow_data_set.get_vocab_transform()
    sentence_ids = vocab_transform(sentence.split(' '))
    sentence_embedding = model.get_embedding(sentence_ids)
    print("这个是句向量的维度：", sentence_embedding.shape)


if __name__ == '__main__':
    main()