BERT微调
微调 BERT
BERT 对每一个词元（ token ）返回抽取了上下文信息的特征向量
不同的任务使用不同的特征
句子分类
将 < cls > 对应的向量输入到全连接层分类
命名实体识别
识别一个词元是不是命名实体，例如人名、机构、位置
将非特殊词元放进全连接层分类
问题问答
给定一个问题，和描述文字，找出一个片段作为回答
对片段中的每个词元预测它是不是回答的开头或结束
总结
即使下游任务各有不同，使用 BERT 微调时均只需要增加输出层
但根据任务的不同，输入的表示，和使用的 BERT 特征也会不一样
自然语言推理数据集
**斯坦福自然语言推断语料库（Stanford Natural Language Inference，SNLI）** \]是由500000多个带标签的英语句子对组成的集合 :cite:`Bowman.Angeli.Potts.ea.2015`。
```python
import os
import re
import torch
from torch import nn
from d2l import torch as d2l
d2l.DATA_HUB['SNLI'] = (
'https://nlp.stanford.edu/projects/snli/snli_1.0.zip',
'9fcde07509c7e87ec61c640c1b2753d9041758e4')
data_dir = d2l.download_extract('SNLI')
```
Reading the Dataset
```python
def read_snli(data_dir, is_train):
"""将SNLI数据集解析为前提、假设和标签"""
def extract_text(s):
# 删除我们不会使用的信息
s = re.sub('\\(', '', s)
s = re.sub('\\)', '', s)
# 用一个空格替换两个或多个连续的空格
s = re.sub('\\s{2,}', ' ', s)
return s.strip()
label_set = {'entailment': 0, 'contradiction': 1, 'neutral': 2}
file_name = os.path.join(data_dir, 'snli_1.0_train.txt'
if is_train else 'snli_1.0_test.txt')
with open(file_name, 'r') as f:
rows = [row.split('\t') for row in f.readlines()[1:]]
premises = [extract_text(row[1]) for row in rows if row[0] in label_set]
hypotheses = [extract_text(row[2]) for row in rows if row[0] \
in label_set]
labels = [label_set[row[0]] for row in rows if row[0] in label_set]
return premises, hypotheses, labels
```
Print the first 3 pairs
```python
train_data = read_snli(data_dir, is_train=True)
for x0, x1, y in zip(train_data[0][:3], train_data[1][:3], train_data[2][:3]):
print('前提：', x0)
print('假设：', x1)
print('标签：', y)
```
Labels " entailment " , " contradiction " , and " neutral " are balanced
```python
test_data = read_snli(data_dir, is_train=False)
for data in [train_data, test_data]:
print([[row for row in data[2]].count(i) for i in range(3)])
# [183416, 183187, 182764]
# [3368, 3237, 3219]
```
Defining a Class for Loading the Dataset
```python
class SNLIDataset(torch.utils.data.Dataset):
"""用于加载SNLI数据集的自定义数据集"""
def __init__(self, dataset, num_steps, vocab=None):
self.num_steps = num_steps
all_premise_tokens = d2l.tokenize(dataset[0])
all_hypothesis_tokens = d2l.tokenize(dataset[1])
if vocab is None:
self.vocab = d2l.Vocab(all_premise_tokens + \
all_hypothesis_tokens, min_freq=5, reserved_tokens=[''])
else:
self.vocab = vocab
self.premises = self._pad(all_premise_tokens)
self.hypotheses = self._pad(all_hypothesis_tokens)
self.labels = torch.tensor(dataset[2])
print('read ' + str(len(self.premises)) + ' examples')
def _pad(self, lines):
return torch.tensor([d2l.truncate_pad(
self.vocab[line], self.num_steps, self.vocab[''])
for line in lines])
def __getitem__(self, idx):
return (self.premises[idx], self.hypotheses[idx]), self.labels[idx]
def __len__(self):
return len(self.premises)
```
Putting All Things Together
```python
def load_data_snli(batch_size, num_steps=50):
"""下载SNLI数据集并返回数据迭代器和词表"""
num_workers = d2l.get_dataloader_workers()
data_dir = d2l.download_extract('SNLI')
train_data = read_snli(data_dir, True)
test_data = read_snli(data_dir, False)
train_set = SNLIDataset(train_data, num_steps)
test_set = SNLIDataset(test_data, num_steps, train_set.vocab)
train_iter = torch.utils.data.DataLoader(train_set, batch_size,
shuffle=True,
num_workers=num_workers)
test_iter = torch.utils.data.DataLoader(test_set, batch_size,
shuffle=False,
num_workers=num_workers)
return train_iter, test_iter, train_set.vocab
```
## BERT微调代码
```python
import json
import multiprocessing
import os
import torch
from torch import nn
from d2l import torch as d2l
```
Loading Pretrained BERT
```python
d2l.DATA_HUB['bert.base'] = (d2l.DATA_URL + 'bert.base.torch.zip',
'225d66f04cae318b841a13d32af3acc165f253ac')
d2l.DATA_HUB['bert.small'] = (d2l.DATA_URL + 'bert.small.torch.zip',
'c72329e68a732bef0452e4b96a1c341c8910f81f')
```
Load pretrained BERT parameters
```python
def load_pretrained_model(pretrained_model, num_hiddens, ffn_num_hiddens,
num_heads, num_layers, dropout, max_len, devices):
data_dir = d2l.download_extract(pretrained_model)
# 定义空词表以加载预定义词表
vocab = d2l.Vocab()
vocab.idx_to_token = json.load(open(os.path.join(data_dir,
'vocab.json')))
vocab.token_to_idx = {token: idx for idx, token in enumerate(
vocab.idx_to_token)}
bert = d2l.BERTModel(len(vocab), num_hiddens, norm_shape=[256],
ffn_num_input=256, ffn_num_hiddens=ffn_num_hiddens,
num_heads=4, num_layers=2, dropout=0.2,
max_len=max_len, key_size=256, query_size=256,
value_size=256, hid_in_features=256,
mlm_in_features=256, nsp_in_features=256)
# 加载预训练BERT参数
bert.load_state_dict(torch.load(os.path.join(data_dir,
'pretrained.params')))
return bert, vocab
devices = d2l.try_all_gpus()
bert, vocab = load_pretrained_model(
'bert.small', num_hiddens=256, ffn_num_hiddens=512, num_heads=4,
num_layers=2, dropout=0.1, max_len=512, devices=devices)
```
The Dataset for Fine-Tuning BERT
```python
class SNLIBERTDataset(torch.utils.data.Dataset):
def __init__(self, dataset, max_len, vocab=None):
all_premise_hypothesis_tokens = [[
p_tokens, h_tokens] for p_tokens, h_tokens in zip(
*[d2l.tokenize([s.lower() for s in sentences])
for sentences in dataset[:2]])]
self.labels = torch.tensor(dataset[2])
self.vocab = vocab
self.max_len = max_len
(self.all_token_ids, self.all_segments,
self.valid_lens) = self._preprocess(all_premise_hypothesis_tokens)
print('read ' + str(len(self.all_token_ids)) + ' examples')
def _preprocess(self, all_premise_hypothesis_tokens):
pool = multiprocessing.Pool(4) # 使用4个进程
out = pool.map(self._mp_worker, all_premise_hypothesis_tokens)
all_token_ids = [
token_ids for token_ids, segments, valid_len in out]
all_segments = [segments for token_ids, segments, valid_len in out]
valid_lens = [valid_len for token_ids, segments, valid_len in out]
return (torch.tensor(all_token_ids, dtype=torch.long),
torch.tensor(all_segments, dtype=torch.long),
torch.tensor(valid_lens))
def _mp_worker(self, premise_hypothesis_tokens):
p_tokens, h_tokens = premise_hypothesis_tokens
self._truncate_pair_of_tokens(p_tokens, h_tokens)
tokens, segments = d2l.get_tokens_and_segments(p_tokens, h_tokens)
token_ids = self.vocab[tokens] + [self.vocab['']] \
* (self.max_len - len(tokens))
segments = segments + [0] * (self.max_len - len(segments))
valid_len = len(tokens)
return token_ids, segments, valid_len
def _truncate_pair_of_tokens(self, p_tokens, h_tokens):
# 为BERT输入中的''、''和''词元保留位置
while len(p_tokens) + len(h_tokens) > self.max_len - 3:
if len(p_tokens) > len(h_tokens):
p_tokens.pop()
else:
h_tokens.pop()
def __getitem__(self, idx):
return (self.all_token_ids[idx], self.all_segments[idx],
self.valid_lens[idx]), self.labels[idx]
def __len__(self):
return len(self.all_token_ids)
```
Generate training and testing examples
```python
# 如果出现显存不足错误，请减少"batch_size"。在原始的BERT模型中，max_len=512
batch_size, max_len, num_workers = 512, 128, d2l.get_dataloader_workers()
data_dir = d2l.download_extract('SNLI')
train_set = SNLIBERTDataset(d2l.read_snli(data_dir, True), max_len, vocab)
test_set = SNLIBERTDataset(d2l.read_snli(data_dir, False), max_len, vocab)
train_iter = torch.utils.data.DataLoader(train_set, batch_size, shuffle=True,
num_workers=num_workers)
test_iter = torch.utils.data.DataLoader(test_set, batch_size,
num_workers=num_workers)
# read 549367 examples
# read 9824 examples
```
This MLP transforms the BERT representation of the special "\< cls \>" token into three outputs of natural language inference
```python
class BERTClassifier(nn.Module):
def __init__(self, bert):
super(BERTClassifier, self).__init__()
self.encoder = bert.encoder
self.hidden = bert.hidden
self.output = nn.Linear(256, 3)
def forward(self, inputs):
tokens_X, segments_X, valid_lens_x = inputs
encoded_X = self.encoder(tokens_X, segments_X, valid_lens_x)
return self.output(self.hidden(encoded_X[:, 0, :]))
net = BERTClassifier(bert)
```
The training
```python
lr, num_epochs = 1e-4, 5
trainer = torch.optim.Adam(net.parameters(), lr=lr)
loss = nn.CrossEntropyLoss(reduction='none')
d2l.train_ch13(net, train_iter, test_iter, loss, trainer, num_epochs,
devices)
```
