本文将介绍大语言模型中使用的不同令牌遮蔽技术，并比较它们的优点，以及使用Pytorch实现以了解它们的底层工作原理。
令牌掩码Token Masking是一种广泛应用于语言模型分类变体和生成模型训练的策略。BERT语言模型首先使用，并被用于许多变体(RoBERTa, ALBERT, DeBERTa…)。
而Text Corruption是一种更大的令牌遮蔽策略。在BART研究论文中，进行了大量实验来训练具有不同策略的编码器-解码器生成模型。
在进入正题之前，我们先介绍大型语言模型(llm)中掩码策略的背景
从监督到自监督
语言模型的初始训练中使用了大量文本，其目标是使模型学会正确地表示语言，并将这种知识隐式地存储在其参数权重中。
大量的文本必须具有用于训练的标签，因为必须在处理模型输入数据并使用参考数据之后计算损失（交叉熵）。但是注释如此大量的数据是不可行的。所以智能将问题从监督学习变为自动生成标签的自监督问题。
在这种情况下，被破坏的文本序列作为模型的训练输入，而所有或部分原始序列作为训练数据的标签。这样通过自动生成的标签，模型学习与每个训练示例关联的标签，就不需要手动的注释数据。
在Text Corruption中（特别是在Token Masking、Token Deletion和Text Infilling中），每个单词可能会按照固定概率（通常约为15-20%）进行遮蔽。这个概率保持较低，以便模型即使在序列被损坏的情况下也能学习每个句子的上下文。
还有一些技术，如Sentence Permutation 或Document Rotation，不会专注于按照一定概率遮蔽单词，我们后面会介绍。
在训练语言模型时，标签会根据是分类模型（仅编码器）还是生成模型（编码器-解码器）而变化。在分类模型中，使用的标签只关注输入中被遮蔽的区域。因此如果一个词在整个句子中被屏蔽，标签只是单个单词。而对于生成模型，由于模型必须能够连续生成文本，输出标签是初始未损坏的序列，关注整个序列本身。
环境配置
我们已经简要介绍了使用Text Corruption训练语言模型的一些背景知识，下面我们开始使用示例代码来介绍不同的Text Corruption技术。
我们将使用Stanza，一个由斯坦福NLP开发的库，其中包含不同的NLP工具，这些工具对我们的预处理非常有用。
import stanza
stanza.download('en')
# Text used in our examples
text = "Huntington's disease is a neurodegenerative autosomal disease
results due to expansion of polymorphic CAG repeats in the huntingtin gene.
Phosphorylation of the translation initiation factor 4E-BP results in the
alteration of the translation control leading to unwanted protein synthesis
and neuronal function. Consequences of mutant huntington (mhtt) gene
transcription are not well known. Variability of age of onset is an
important factor of Huntington's disease separating adult and juvenile types.
The factors which are taken into account are-genetic modifiers, maternal
protection i.e excessive paternal transmission, superior ageing genes
and environmental threshold. A major focus has been given to the molecular
pathogenesis which includes-motor disturbance, cognitive disturbance and
neuropsychiatric disturbance. The diagnosis part has also been taken care of.
This includes genetic testing and both primary and secondary symptoms.
The present review also focuses on the genetics and pathology of Huntington's
disease."
# We will use a stanza model for getting each different sentence
# as an element of the list
nlp = stanza.Pipeline('en', use_gpu=False)
doc = nlp(text)
sentences = [sentence.text for sentence in doc.sentences]
Token Masking
令牌掩码用替换文本中的随机单词
这是从BERT引入的策略，它包括通过屏蔽随机单词来破坏输入序列，这些单词将在训练期间用作输出标签。
在分类模型中，我们可以直接使用Huggingface的DataCollatorForLanguageModeling类来生成必要的标签，这样就可以训练像BERT或RoBERTa这样的模型。
from transformers import AutoTokenizer, DataCollatorForLanguageModeling
import torch
def load_dataset_mlm(sentences, tokenizer_class=AutoTokenizer,
collator_class=DataCollatorForLanguageModeling,
mlm=True, mlm_probability=0.20):
tokenizer = tokenizer_class.from_pretrained('google-bert/bert-base-uncased')
inputs = tokenizer(sentences, return_tensors='pt', padding=True,
truncation=True)
# Random masking configuration
data_collator = collator_class(
tokenizer=tokenizer,
mlm=mlm,
mlm_probability=mlm_probability
)
"""The collator expects a tuple of tensors, so you have to split
the input tensors and then remove the first dimension and pass it
to a tuple. """
tuple_ids = torch.split(inputs['input_ids'], 1, dim=0)
tuple_ids = list(tuple_ids)
for tensor in range(len(tuple_ids)):
tuple_ids[tensor] = tuple_ids[tensor].squeeze(0)
tuple_ids = tuple(tuple_ids)
# Get input_ids, attention_masks and labels for each sentence.
batch = data_collator(tuple_ids)
return batch['input_ids'], inputs['attention_mask'], batch['labels']
input_ids, attention_mask, labels = load_dataset_mlm(sentences)
"""
input_ids[0]:
tensor([ 101, 16364, 1005, 1055, 103, 2003, 1037, 103, 10976, 3207,
103, 25284, 103, 25426, 16870, 4295, 3463, 2349, 2000, 103,
1997, 26572, 18078, 6187, 2290, 17993, 1999, 1996, 5933, 7629,
103, 103, 102, 0, 0])
attention_mask[0]:
tensor([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0])
labels[0]:
tensor([ -100, -100, -100, -100, 4295, -100, -100, 11265, -100, -100,
6914, -100, 8285, -100, 2389, -100, -100, -100, -100, 4935,
-100, -100, -100, -100, -100, -100, -100, -100, -100, -100,
4962, 1012, -100, -100, -100]))
"""
生成的inputs_ids对原始文本的每个标记都是整数。一个特殊的标记表示被屏蔽的单词(在BERT中，这个标记是103)。这个特殊的标记根据所使用的语言模型而变化，因此不同的标记器将返回注意掩码的不同标识符。
Huggingface还在模型中使用不同的操作分配唯一的令牌，因此用“-100”表示的令牌表示模型应该忽略它们。
对于像BART这样的生成模型，我们可以使用DataCollatorForLanguageModeling类实现令牌屏蔽策略。但是需要一些小的更改，以使标记适应生成模型。
from transformers import BartTokenizer, DataCollatorForLanguageModeling
import torch
def load_dataset_mlm(sentences, tokenizer_class=BartTokenizer,
collator_class=DataCollatorForLanguageModeling,
mlm=True, mlm_probability=0.20):
tokenizer = tokenizer_class.from_pretrained('facebook/bart-base')
inputs = tokenizer(sentences, return_tensors='pt', padding=True,
truncation=True)
# Random masking configuration
data_collator = collator_class(
tokenizer=tokenizer,
mlm=mlm, # True for Masked Language Modelling
mlm_probability=mlm_probability # Chance for every token to get masked
)
"""The collator expects a tuple of tensors, so you have to split
the input tensors and then remove the first dimension and pass it
to a tuple. """
tuple_ids = torch.split(inputs['input_ids'], 1, dim=0)
tuple_ids = list(tuple_ids)
for tensor in range(len(tuple_ids)):
tuple_ids[tensor] = tuple_ids[tensor].squeeze(0)
tuple_ids = tuple(tuple_ids)
# Get input_ids, attention_masks and labels for each sentence.
batch = data_collator(tuple_ids)
batch['labels'] = inputs['input_ids']
return batch['input_ids'], inputs['attention_mask'], batch['labels']
input_ids, attention_mask, labels = load_dataset_mlm(sentences)
"""
input_ids[0]:
tensor([ 0, 38831, 2577, 1054, 18, 2199, 16, 10, 14913, 28904,
5777, 3693, 32226, 38868, 2199, 775, 528, 7, 2919, 9,
48052, 636, 230, 3450, 35315, 11, 5, 50264, 50264, 50264,
4, 2])
attention_mask[0]:
tensor([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1])
labels[0]:
tensor([ 0, 38831, 2577, 1054, 18, 2199, 16, 10, 14913, 28904,
5777, 3693, 32226, 38868, 2199, 775, 528, 7, 2919, 9,
48052, 636, 230, 3450, 35315, 11, 5, 8217, 24276, 10596,
4, 2])
"""
每个输入标记都标记着与之对应的标记，无论它是否被屏蔽。这是因为与分类模型不同，模型必须能够基于给定给模型的序列生成文本序列。在BART的情况下，表示屏蔽的标记的ID是50264。
Token Deletion
使用标记删除 Token Deletion，模型必须学习确切的位置和缺失的词是什么，因此它必须比仅使用Token Masking学习更多的特征。
这种策略使用了一种不同的屏蔽方法。以一定的概率一个词从原始文本序列中被移除，因此模型必须找到缺失的单词及其位置。标准的屏蔽方法不会学习位置，因为屏蔽已经在模型的输入中指示
def token_deletion(sentences, tokenizer_class=BartTokenizer, collator_class=DataCollatorForLanguageModeling,
mlm=True, mlm_probability=0.20):
tokenizer = tokenizer_class.from_pretrained('facebook/bart-base')
inputs = tokenizer(sentences, return_tensors='pt', padding=True, truncation=True)
data_collator = collator_class(
tokenizer=tokenizer,
mlm=mlm,
mlm_probability=mlm_probability
)
tuple_ids = torch.split(inputs['input_ids'], 1, dim=0)
tuple_ids = list(tuple_ids)
for tensor in range(len(tuple_ids)):
tuple_ids[tensor] = tuple_ids[tensor].squeeze(0)
tuple_ids = tuple(tuple_ids)
batch = data_collator(tuple_ids)
# We use the initial inputs as labels
batch['labels'] = batch['input_ids'].clone()
# We remove tokens with mask identifier and thus make token deletion
# Change the value to the mask identifier of the specific token model
# It is necessary to know the identifier of the mask token for
# that specific model
mask = batch['input_ids'] != 50264
initial_size = batch['input_ids'].size(1)
total_sentences = batch['input_ids'].size(0)
# When we remove the specific token, we must fill with the padding
# token otherwise the tensor size is not respected.
for i in range(total_sentences):
new_tensor = batch['input_ids'][i][mask[i]]
new_tensor = F.pad(new_tensor, (0, initial_size - new_tensor.size(0)), value=1)
batch['input_ids'][i] = new_tensor
attention_mask = batch['input_ids'][i] == 1
inputs['attention_mask'][i][attention_mask] = 0
return batch['input_ids'], inputs['attention_mask'], batch['labels']
input_ids, attention_mask, labels = token_deletion(sentences)
"""
input_ids[0]:
tensor([ 0, 38831, 2577, 1054, 2199, 14913, 28904, 3693, 32226, 38868,
2199, 775, 528, 7, 2919, 9, 23404, 636, 230, 35315,
11, 5, 24276, 10596, 4, 2, 1, 1, 1, 1,
1, 1])
attention_mask[0]:
tensor([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 0, 0, 0, 0, 0, 0])
labels[0]:
tensor([ 0, 38831, 2577, 1054, 50264, 2199, 50264, 50264, 14913, 28904,
50264, 3693, 32226, 38868, 2199, 775, 528, 7, 2919, 9,
23404, 636, 230, 50264, 35315, 11, 5, 50264, 24276, 10596,
4, 2])
"""
当使用Token Deletion训练BART时，长序列用于问答、摘要生成任务和会话任务会有一定的提高。
Text Infilling
文本填充 Text Infilling允许模型学习每个屏蔽位置可以有多少个单词。而先前的方法假设每个屏蔽位置只有一个单词。
Text Infilling与Token Masking类似，因为我们会以一定的概率在原始文本上使用屏蔽。但是不同之处在于屏蔽可以覆盖多个单词。在BART中，屏蔽是用泊松分布 lambda = 3 进行的；这意味着平均而言，每次对句子中的文本进行屏蔽时，会有三个单词被包含在一个单个的标记中，但由于这是一个概率分布，可能会有更多或更少的屏蔽单词。
我们将使用Numpy库和特定于我们的语言模型(在本例中是BART)的标记器来实现文本填充。
import numpy as np
from transformers import BartTokenizer
def text_infilling(sentence, probability=0.2, poisson_lambda=3):
# We'll use a binary mask to determine which words to replace
mask = np.random.choice([0, 1], size=len(sentence), p=[1-probability, probability])
# Now we'll replace the chosen words with a mask token
# We'll also use a Poisson distribution to determine the length of the spans to mask
for i in range(len(mask)):
if mask[i] == 1:
span_length = np.random.poisson(poisson_lambda)
for j in range(span_length):
if i + j < len(sentence):
sentence[i + j] = "
"
infilled_sentence = []
for token in range(len(sentence)):
if sentence[token] == "
":
if token < len(sentence)-1:
if sentence[token+1] == "
":
continue
else:
infilled_sentence.append(sentence[token])
else:
infilled_sentence.append(sentence[token])
else:
infilled_sentence.append(sentence[token])
return " ".join(infilled_sentence)
def text_infilling_input(masked_sentences, sentences, tokenizer_class=BartTokenizer):
tokenizer = tokenizer_class.from_pretrained('facebook/bart-base')
inputs = tokenizer(masked_sentences, return_tensors='pt', padding=True, truncation=True)
labels = tokenizer(sentences, return_tensors='pt', padding=True, truncation=True)
return inputs['input_ids'], inputs['attention_mask'], labels['input_ids']
input_ids, attention_mask, labels = text_infilling_input(masked_sentences, sentences)
"""
input_ids[0]:
tensor([ 0, 50264, 16, 50264, 2199, 775, 528, 50264, 48052, 636,
50264, 8217, 24276, 10596, 4, 2, 1, 1, 1, 1,
1, 1, 1])
attention_mask[0]:
tensor([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0])
labels[0]:
tensor([ 0, 38831, 2577, 1054, 18, 2199, 16, 10, 14913, 28904,
5777, 3693, 32226, 38868, 2199, 775, 528, 7, 2919, 9,
48052, 636, 230, 3450, 35315, 11, 5, 8217, 24276, 10596,
4, 2])
"""
Text Infilling比Token Deletion更能改善BART语言模型的结果，在问题回答、文本摘要和会话任务中提供更好的生成。
Sentence Permutation
语言模型的输入文本被分成随机重新排列的句子，模型需要找出原始的顺序。
在Sentence Permutation中，考虑适合模型输入序列的句子数量是至关重要的(在小型模型中，输入序列在512到1024之间)。在确定符合序列的句子数量之后，需要将它们分离到一个列表或数组中，并随机选择，而不重复其中任何一个。
# It selects the first "number_sentences" within a given set of "sentences"
# and returns those sentences in a random order.
def sentence_permutation(sentences, number_sentences):
new_sentences = sentences[:number_sentences]
random.shuffle(new_sentences)
new_sentences = sentence_joiner(new_sentences)
return new_sentences
def permuted_data_generation(sentences: list, total_sentences: int):
training_sentences = []
training_labels = []
sentences_copy = sentences.copy()
# We can apply sentence_permutation a number of times equal to the
# size of the list - 1 to get an example with each new sentence in
# the text, removing the oldest one.
for _ in range(len(sentences)-total_sentences+1):
new_sentences = sentence_permutation(sentences_copy, total_sentences)
joined_sentences = sentence_joiner(sentences_copy[:total_sentences])
sentences_copy = sentences_copy[1:]
training_sentences.append(new_sentences)
training_labels.append(joined_sentences)
return training_sentences, training_labels
def permutation_training(sentences: list, sentences_labels: list,
tokenizer_class=BartTokenizer,
collator_class=DataCollatorForLanguageModeling,
mlm=True, mlm_probability=0.0):
# We get input_ids and attention mask from the permuted sentences
input, attention_mask, _ = load_dataset_mlm(sentences, tokenizer_class, collator_class, mlm, mlm_probability)
# Labels from the original sentences
labels, _, _ = load_dataset_mlm(sentences_labels, tokenizer_class, collator_class, mlm, mlm_probability)
return input.squeeze(0), attention_mask.squeeze(0), labels.squeeze(0)
input_ids, attention_mask, labels = permutation_training(training_sentences, training_labels_sentences)
"""
input_ids[0]:
tensor([ 0, 38831, 2577, 1054, 18, 2199, 16, 10, 14913, 28904,
5777, 3693, 32226, 38868, 2199, 775, 528, 7, 2919, 9,
48052, 636, 230, 3450, 35315, 11, 5, 8217, 24276, 10596,
4, 2585, 33430, 8457, 9, 41419, 8217, 1054, 36, 119,
49491, 43, 10596, 37118, 32, 45, 157, 684, 4, 4129,
33839, 4405, 35019, 9, 5, 19850, 34939, 3724, 204, 717,
12, 21792, 775, 11, 5, 39752, 9, 5, 19850, 797,
981, 7, 15067, 8276, 37423, 8, 46282, 5043, 4, 2])
attention_mask[0]:
tensor([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1])
labels[0]:
tensor([ 0, 38831, 2577, 1054, 18, 2199, 16, 10, 14913, 28904,
5777, 3693, 32226, 38868, 2199, 775, 528, 7, 2919, 9,
48052, 636, 230, 3450, 35315, 11, 5, 8217, 24276, 10596,
4, 4129, 33839, 4405, 35019, 9, 5, 19850, 34939, 3724,
204, 717, 12, 21792, 775, 11, 5, 39752, 9, 5,
19850, 797, 981, 7, 15067, 8276, 37423, 8, 46282, 5043,
4, 2585, 33430, 8457, 9, 41419, 8217, 1054, 36, 119,
49491, 43, 10596, 37118, 32, 45, 157, 684, 4, 2])
"""
我们对于模型的每个数据输入，删除原始序列中出现的第一个句子，然后在执行基于要选择的固定句子数目的句子排列之前，将接下来的句子添加进去。这样虽然重新排列了输入序列中的句子，但保持了一个每个新例子中都会出现一个新的句子，并删除最旧的句子的上下文窗口。
Document Rotation
当旋转一个文档时，选择一个特定的词，并将其设定为起始词，而所有之前的词都被粘贴到文本的末尾。
如果要应用Document Rotation，必须考虑到每个批次使用的维度。在应用填充的情况下，这个填充不能与文档的其余部分一起旋转，而是必须保持其原始位置，同时整个文档旋转。
def sentence_joiner(sentences: list):
return ' '.join(sentences)
# With this function we gather as many sentences as we want to form the input data to the tokenizer.
def rotated_data_generation(sentences: list, total_sentences: int):
training_sentences = []
sentences_copy = sentences.copy()
for _ in range(len(sentences)-total_sentences+1):
new_sentences = sentences_copy[:total_sentences]
new_sentences = sentence_joiner(new_sentences)
sentences_copy = sentences_copy[1:]
training_sentences.append(new_sentences)
return training_sentences
# Apply this function over the rotated sentences from previous function
def document_rotation_training(sentences, tokenizer_class=BartTokenizer):
tokenizer = tokenizer_class.from_pretrained('facebook/bart-base')
tokens = tokenizer(sentences, return_tensors='pt', padding=True, truncation=True)
tokens['input_ids'] = tokens['input_ids'].squeeze(0)
tokens['labels'] = tokens['input_ids'].clone()
iterations = tokens['input_ids'].size(0)
for i in range(iterations):
# Get the attention mask and convert to list
attention_mask = tokens['attention_mask'][i].tolist()
# Calculate the position where padding starts
if 0 in attention_mask:
padding_start_position = attention_mask.index(0)
else:
padding_start_position = False
# We take into account the position of the padding so as not to rotate it along with the rest of the document.
if padding_start_position:
random_token = torch.randint(1, padding_start_position-1, (1,))
tokens['input_ids'][i] = torch.cat((tokens['input_ids'][i][0].unsqueeze(0), #initial token
tokens['input_ids'][i][random_token.item():padding_start_position-1], #from random to padding
tokens['input_ids'][i][1:random_token.item()], #from 1 to random
tokens['input_ids'][i][padding_start_position-1:-1],
tokens['input_ids'][i][-1].unsqueeze(0)), 0)
# If there is no padding, we rotate the document without taking the padding into account.
else:
random_token = torch.randint(1, tokens['input_ids'].size(0)-1, (1,))
tokens['input_ids'][i] = torch.cat((tokens['input_ids'][i][0].unsqueeze(0), #initial token
tokens['input_ids'][i][random_token.item():-1], #from random to end
tokens['input_ids'][i][1:random_token.item()],
tokens['input_ids'][i][-1].unsqueeze(0)), 0)
return tokens['input_ids'], tokens['attention_mask'].squeeze(0), tokens['labels']
data = rotated_data_generation(sentences, 3)
input_ids, attention_mask, labels = document_rotation_training(data)
"""
input_ids[2]:
tensor([ 0, 2433, 61, 32, 551, 88, 1316, 32, 12, 4138,
15557, 47605, 6, 22835, 2591, 939, 4, 242, 10079, 38422,
9235, 6, 10295, 22540, 14819, 8, 3039, 11543, 4, 347,
37347, 8457, 9, 41419, 8217, 1054, 36, 119, 49491, 43,
10596, 37118, 32, 45, 157, 684, 4, 41058, 4484, 9,
1046, 9, 23808, 16, 41, 505, 3724, 9, 18073, 18,
2199, 18246, 4194, 8, 13430, 3505, 4, 20, 2, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1])
attention_mask[2]:
tensor([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0])
labels[2]:
tensor([ 0, 347, 37347, 8457, 9, 41419, 8217, 1054, 36, 119,
49491, 43, 10596, 37118, 32, 45, 157, 684, 4, 41058,
4484, 9, 1046, 9, 23808, 16, 41, 505, 3724, 9,
18073, 18, 2199, 18246, 4194, 8, 13430, 3505, 4, 20,
2433, 61, 32, 551, 88, 1316, 32, 12, 4138, 15557,
47605, 6, 22835, 2591, 939, 4, 242, 10079, 38422, 9235,
6, 10295, 22540, 14819, 8, 3039, 11543, 4, 2, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1])
"""
类似于序列排列，我们可以在每个数据输入中移除最旧的句子，并添加一个新句子，从而保持上下文窗口。
总结
本文介绍了讨论了训练语言模型的不同的令牌掩码。虽然这些都是比较常见的方法，但是大多数模型只使用了Token Masking。
对于短文本序列来说，Sentence Permutation 和Document Rotation技术可能没有帮助甚至会降低准确率。而Token Masking、Token Deletion和Text Infilling 在短文本和长文本序列中都可以使用。
https://avoid.overfit.cn/post/1b9d2c9d6b9a4bacbe6fa906c23aee7f
作者：Fabio Yáñez Romero