Transformers的微调训练

数据集准备和预处理

通过dataset包加载数据集
定义Dataset.map要使用的预处理函数
定义DataCollator来用于构造训练batch

加载预训练模型

随机初始化ForSequenceClassificationHead

微调训练

Trainer是Huggingface transformers库的一个高级API，可以帮助我们快速搭建训练框架。
默认情况下，Trainer和TrainingArguments会使用：

batch size=8
epochs = 3
AdamW优化器

可以提供一个compute_metrics函数，用于输出我们希望有的一些指标。

import os
import torch
import numpy as np

os.environ['HF_ENDPOINT'] = 'https://hf-mirror.com'

from transformers import AdamW, AutoTokenizer, AutoModelForSequenceClassification

# Same as before
checkpoint = "bert-base-uncased"
tokenizer = AutoTokenizer.from_pretrained(checkpoint)
model = AutoModelForSequenceClassification.from_pretrained(checkpoint, num_labels=2)

sequences = [
    "I've been waiting for a HuggingFace course my whole life.",
    "This course is amazing!",
]
batch = tokenizer(sequences, padding=True, truncation=True, return_tensors="pt")
batch['labels'] = torch.tensor([1, 1])  # tokenizer出来的结果是一个dictionary，所以可以直接加入新的 key-value

optimizer = AdamW(model.parameters())
loss = model(**batch).loss  #这里的 loss 是直接根据 batch 中提供的 labels 来计算的，回忆：前面章节查看 model 的输出的时候，有loss这一项
loss.backward()
optimizer.step()

from datasets import load_dataset

raw_datasets = load_dataset("glue", "mrpc")
raw_train_dataset = raw_datasets['train']

tokenized_sentences_1 = tokenizer(raw_train_dataset['sentence1'])
tokenized_sentences_2 = tokenizer(raw_train_dataset['sentence2'])

from pprint import pprint as print
inputs = tokenizer("first sentence", "second one")
print(inputs)

def tokenize_function(sample):
    # 这里可以添加多种操作，不光是tokenize
    # 这个函数处理的对象，就是Dataset这种数据类型，通过features中的字段来选择要处理的数据
    return tokenizer(sample['sentence1'], sample['sentence2'], truncation=True)

# Dataset.map节约内存
tokenized_datasets = raw_datasets.map(tokenize_function, batched=True)
print(tokenized_datasets)

# 划分batch的时候再进行padding
from transformers import DataCollatorWithPadding
data_collator = DataCollatorWithPadding(tokenizer=tokenizer)

samples = tokenized_datasets['train'][:5]
print(samples.keys())

# 把这里多余的几列去掉
samples = {k:v for k,v in samples.items() if k not in ["idx", "sentence1", "sentence2"]}  # 把这里多余的几列去掉
print(samples.keys())

# 打印出每个句子的长度：
print([len(x) for x in samples["input_ids"]])

# 然后我们使用data_collator来处理padding
batch = data_collator(samples)  # samples中必须包含 input_ids 字段，因为这就是collator要处理的对象
print(batch.keys())

# 再打印长度：
print([len(x) for x in batch['input_ids']])

from transformers import Trainer, TrainingArguments

from datasets import load_metric
def compute_metrics(eval_preds):
    metric = load_metric("glue", "mrpc")
    logits, labels = eval_preds.predictions, eval_preds.label_ids
    # 上一行可以直接简写成：
    # logits, labels = eval_preds  因为它相当于一个tuple
    predictions = np.argmax(logits, axis=-1)
    return metric.compute(predictions=predictions, references=labels)

training_args = TrainingArguments(output_dir='test_trainer') # 指定输出文件夹，没有会自动创建

trainer = Trainer(
    model,
    training_args,
    train_dataset=tokenized_datasets["train"],
    eval_dataset=tokenized_datasets["validation"],
    data_collator=data_collator,  # 在定义了tokenizer之后，其实这里的data_collator就不用再写了，会自动根据tokenizer创建
    tokenizer=tokenizer,
    compute_metrics=compute_metrics
)

# 启动训练
trainer.train()

# trainer.save_model()

# 或者加载之前的训练结果

from transformers.trainer_utils import EvalPrediction, get_last_checkpoint

#last_checkpoint = get_last_checkpoint('test_trainer')
#trainer.train(resume_from_checkpoint=last_checkpoint)

参考资料

Huggingface🤗NLP笔记7：使用Trainer API来微调模型