BERT 获取句子嵌入

Question

我正在复制此页面的代码。我已将 BERT 模型下载到本地系统并获取句子嵌入。

我有大约 500,000 个句子需要句子嵌入，这需要花费很多时间。

1. 有没有办法加快这个过程？
2. 发送一批句子而不是一次发送一个句子会有帮助吗？

。

#!pip install transformers
import torch
import transformers
from transformers import BertTokenizer, BertModel
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
model = BertModel.from_pretrained('bert-base-uncased',
                                  output_hidden_states = True, # Whether the model returns all hidden-states.
                                  )

# Put the model in "evaluation" mode, meaning feed-forward operation.
model.eval()

corpa=["i am a boy","i live in a city"]



storage=[]#list to store all embeddings

for text in corpa:
    # Add the special tokens.
    marked_text = "[CLS] " + text + " [SEP]"

    # Split the sentence into tokens.
    tokenized_text = tokenizer.tokenize(marked_text)

    # Map the token strings to their vocabulary indeces.
    indexed_tokens = tokenizer.convert_tokens_to_ids(tokenized_text)

    segments_ids = [1] * len(tokenized_text)

    tokens_tensor = torch.tensor([indexed_tokens])
    segments_tensors = torch.tensor([segments_ids])

    # Run the text through BERT, and collect all of the hidden states produced
    # from all 12 layers. 
    with torch.no_grad():

        outputs = model(tokens_tensor, segments_tensors)

        # Evaluating the model will return a different number of objects based on 
        # how it's  configured in the `from_pretrained` call earlier. In this case, 
        # becase we set `output_hidden_states = True`, the third item will be the 
        # hidden states from all layers. See the documentation for more details:
        # https://huggingface.co/transformers/model_doc/bert.html#bertmodel
        hidden_states = outputs[2]


    # `hidden_states` has shape [13 x 1 x 22 x 768]

    # `token_vecs` is a tensor with shape [22 x 768]
    token_vecs = hidden_states[-2][0]

    # Calculate the average of all 22 token vectors.
    sentence_embedding = torch.mean(token_vecs, dim=0)

    storage.append((text,sentence_embedding))

######更新1

我根据提供的答案修改了我的代码。它没有进行完整的批处理

#!pip install transformers
import torch
import transformers
from transformers import BertTokenizer, BertModel
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
model = BertModel.from_pretrained('bert-base-uncased',
                                  output_hidden_states = True, # Whether the model returns all hidden-states.
                                  )

# Put the model in "evaluation" mode, meaning feed-forward operation.
model.eval()

batch_sentences = ["Hello I'm a single sentence",
                    "And another sentence",
                    "And the very very last one"]
encoded_inputs = tokenizer(batch_sentences)


storage=[]#list to store all embeddings
for i,text in enumerate(encoded_inputs['input_ids']):
    
    tokens_tensor = torch.tensor([encoded_inputs['input_ids'][i]])
    segments_tensors = torch.tensor([encoded_inputs['attention_mask'][i]])
    print (tokens_tensor)
    print (segments_tensors)

    # Run the text through BERT, and collect all of the hidden states produced
    # from all 12 layers. 
    with torch.no_grad():

        outputs = model(tokens_tensor, segments_tensors)

        # Evaluating the model will return a different number of objects based on 
        # how it's  configured in the `from_pretrained` call earlier. In this case, 
        # becase we set `output_hidden_states = True`, the third item will be the 
        # hidden states from all layers. See the documentation for more details:
        # https://huggingface.co/transformers/model_doc/bert.html#bertmodel
        hidden_states = outputs[2]


    # `hidden_states` has shape [13 x 1 x 22 x 768]

    # `token_vecs` is a tensor with shape [22 x 768]
    token_vecs = hidden_states[-2][0]

    # Calculate the average of all 22 token vectors.
    sentence_embedding = torch.mean(token_vecs, dim=0)
    print (sentence_embedding[:10])
    storage.append((text,sentence_embedding))

我可以将 for 循环中的前两行更新到下面。但只有当所有句子在标记化后具有相同的长度时它们才起作用

tokens_tensor = torch.tensor([encoded_inputs['input_ids']])
segments_tensors = torch.tensor([encoded_inputs['attention_mask']])

而且在这种情况下outputs = model(tokens_tensor, segments_tensors) 会失败。

在这种情况下我怎样才能完全执行批处理呢？

Answer 1

可以加速工作流程的最简单方法之一是批量数据处理。在当前的实现中，您在每次迭代时仅输入一个句子，但可以使用批量数据！

现在，如果您愿意自己实现这部分，我强烈建议您使用tokenizer这种方式来准备数据。

batch_sentences = ["Hello I'm a single sentence",
                    "And another sentence",
                    "And the very very last one"]
encoded_inputs = tokenizer(batch_sentences)
print(encoded_inputs)
{'input_ids': [[101, 8667, 146, 112, 182, 170, 1423, 5650, 102],
               [101, 1262, 1330, 5650, 102],
               [101, 1262, 1103, 1304, 1304, 1314, 1141, 102]],
 'token_type_ids': [[0, 0, 0, 0, 0, 0, 0, 0, 0],
                    [0, 0, 0, 0, 0],
                    [0, 0, 0, 0, 0, 0, 0, 0]],
 'attention_mask': [[1, 1, 1, 1, 1, 1, 1, 1, 1],
                    [1, 1, 1, 1, 1],
                    [1, 1, 1, 1, 1, 1, 1, 1]]}

但有一个更简单的方法，使用FeatureExtractionPipeline全面的文档！这看起来像这样：

from transformers import pipeline

feature_extraction = pipeline('feature-extraction', model="distilroberta-base", tokenizer="distilroberta-base")
features = feature_extraction(["Hello I'm a single sentence",
                               "And another sentence",
                               "And the very very last one"])

UPDATE1 事实上，您稍微更改了代码，但您一次传递一个示例，而不是以批处理形式。如果我们想坚持你的实现批处理将是这样的：

tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
model = BertModel.from_pretrained('bert-base-uncased',
                                  output_hidden_states = True, # Whether the model returns all hidden-states.
                                  )
model.eval()
sentences = [ 
              "Hello I'm a single sentence",
              "And another sentence",
              "And the very very last one",
              "Hello I'm a single sentence",
              "And another sentence",
              "And the very very last one",
              "Hello I'm a single sentence",
              "And another sentence",
              "And the very very last one",
            ]
batch_size = 4  
for idx in range(0, len(sentences), batch_size):
    batch = sentences[idx : min(len(sentences), idx+batch_size)]
    
    # encoded = tokenizer(batch)
    encoded = tokenizer.batch_encode_plus(batch,max_length=50, padding='max_length', truncation=True)
  
    encoded = {key:torch.LongTensor(value) for key, value in encoded.items()}
    with torch.no_grad():
        
        outputs = model(**encoded)
        
    
    print(outputs.last_hidden_state.size())

输出：

torch.Size([4, 50, 768]) # batch_size * max_length * hidden dim
torch.Size([4, 50, 768])
torch.Size([1, 50, 768]) 

更新2

关于将批处理数据填充到最大长度所提到的内容，存在两个问题。第一，它是否能够用不相关的信息来破坏变压器模型？不，因为在训练阶段模型已经以批量的形式呈现了可变长度的输入句子，并且设计者引入了特定的参数来指导模型应该注意的地方！其次，如何摆脱这些垃圾数据？使用该attention mask参数，您可以仅对相关数据执行均值运算！

所以代码应该改成这样：

for idx in range(0, len(sentences), batch_size):
    batch = sentences[idx : min(len(sentences), idx+batch_size)]
    
    # encoded = tokenizer(batch)
    encoded = tokenizer.batch_encode_plus(batch,max_length=50, padding='max_length', truncation=True)
  
    encoded = {key:torch.LongTensor(value) for key, value in encoded.items()}
    with torch.no_grad():
        
        outputs = model(**encoded)
    lhs = outputs.last_hidden_state
    attention = encoded['attention_mask'].reshape((lhs.size()[0], lhs.size()[1], -1)).expand(-1, -1, 768)
    embeddings = torch.mul(lhs, attention)
    denominator = torch.count_nonzero(embeddings, dim=1)
    summation = torch.sum(embeddings, dim=1)
    mean_embeddings = torch.div(summation, denominator)

Answer 2

关于你原来的问题：你无能为力。BERT 是一种计算要求很高的算法。最好的办法是使用BertTokenizerFast而不是常规的BertTokenizer. “快速”版本效率更高，您会看到大量文本的差异。

话虽如此，我必须警告您，平均 BERT 词嵌入并不能为句子创建良好的嵌入。请参阅这篇文章。根据您的问题，我假设您想要进行某种语义相似性搜索。尝试使用其中一种开源模型。