https://www.bilibili.com/video/BV1K5411t7MD?p=5
https://www.youtube.com/channel/UCoRX98PLOsaN8PtekB9kWrw/videos
深入BERT实战(PyTorch) by ChrisMcCormickAI
这是ChrisMcCormickAI在油管bert，8集系列第三篇fine-Tuning的pytorch的讲解的代码，在油管视频下有cloab地址，如果不能翻墙的可以留下邮箱我全部看完整理后发给你。但是在fine-tuning最好还是在cloab上运行

文章目录

深入Bert实战(Pytorch)----fine-Tuning 2
4. Train Our Classification Model
- 4.1. BertForSequenceClassification
- 4.2. Optimizer & Learning Rate Scheduler
- 4.3. 循环训练
5. 在测试集上的表现
- 5.1. 数据准备
- 5.2. 在测试集上评估
总结
附录
- A1. Saving & Loading Fine-Tuned Model
Revision History

4. Train Our Classification Model

4.1. BertForSequenceClassification

对于这个任务，我们首先要修改预训练的BERT模型以给出分类输出，然后在自己的数据集上继续训练模型，直到整个模型(端到端的模型)非常适合自己的任务。

值得庆幸的是，huggingface pytorch实现包含一组为各种NLP任务设计的接口。尽管这些接口都建立在训练好的BERT模型之上，但每个接口都有不同的顶层和输出类型，以适应它们特定的NLP任务。

这里是目前提供的fine-tuning列表

BertModel
BertForPreTraining
BertForMaskedLM
BertForNextSentencePrediction
BertForSequenceClassification - The one we’ll use.
BertForTokenClassification
BertForQuestionAnswering

这里是transformer的文档here.

我们使用BertForSequenceClassification。这是普通的BERT模型，上面添加了一个用于分类的线性层，我们将使用它作为句子分类器。当我们输入数据时，整个预训练的BERT模型和额外的未训练的分类层是同时在这个任务上进行训练

好的，现在加载BERT！这里有几种不同的预训练模型，"bert-base-uncased"版本，仅有小写字母(“uncased”)相比于是较小的(“base” vs “large”)。

预训练的文档在from_pretrainedhere 定义了其它参数 here

from transformers import BertForSequenceClassification, AdamW, BertConfig# Load BertForSequenceClassification, the pretrained BERT model with a single
# linear classification layer on top.
# 加载BertForSequenceClassification，预训练的模型+顶层单层线性分类层
model = BertForSequenceClassification.from_pretrained("bert-base-uncased", # Use the 12-layer BERT model, with an uncased vocab.num_labels = 2, # The number of output labels--2 for binary classification.# You can increase this for multi-class tasks.# 2分类问题，可以增加为多分类问题output_attentions = False, # Whether the model returns attentions weights.output_hidden_states = False, # Whether the model returns all hidden-states.
)# Tell pytorch to run this model on the GPU.
model.cuda()

出于好奇，我们可以在这里按名称浏览所有的模型参数。
在下面的单元格中，我打印出了以下权重的名称和尺寸:

这里作者打印了所有层，总共有201层，也打印了权重和大小

The embedding layer.
The first of the twelve transformers.
The output layer.

# Get all of the model's parameters as a list of tuples.
params = list(model.named_parameters())print('The BERT model has {:} different named parameters.\n'.format(len(params)))print('==== Embedding Layer ====\n')for p in params[0:5]:print("{:<55} {:>12}".format(p[0], str(tuple(p[1].size()))))print('\n==== First Transformer ====\n')for p in params[5:21]:print("{:<55} {:>12}".format(p[0], str(tuple(p[1].size()))))print('\n==== Output Layer ====\n')for p in params[-4:]:print("{:<55} {:>12}".format(p[0], str(tuple(p[1].size()))))

The BERT model has 201 different named parameters.==== Embedding Layer ====bert.embeddings.word_embeddings.weight                  (30522, 768)
bert.embeddings.position_embeddings.weight                (512, 768)
bert.embeddings.token_type_embeddings.weight                (2, 768)
bert.embeddings.LayerNorm.weight                              (768,)
bert.embeddings.LayerNorm.bias                                (768,)==== First Transformer ====bert.encoder.layer.0.attention.self.query.weight          (768, 768)
bert.encoder.layer.0.attention.self.query.bias                (768,)
bert.encoder.layer.0.attention.self.key.weight            (768, 768)
bert.encoder.layer.0.attention.self.key.bias                  (768,)
bert.encoder.layer.0.attention.self.value.weight          (768, 768)
bert.encoder.layer.0.attention.self.value.bias                (768,)
bert.encoder.layer.0.attention.output.dense.weight        (768, 768)
bert.encoder.layer.0.attention.output.dense.bias              (768,)
bert.encoder.layer.0.attention.output.LayerNorm.weight        (768,)
bert.encoder.layer.0.attention.output.LayerNorm.bias          (768,)
bert.encoder.layer.0.intermediate.dense.weight           (3072, 768)
bert.encoder.layer.0.intermediate.dense.bias                 (3072,)
bert.encoder.layer.0.output.dense.weight                 (768, 3072)
bert.encoder.layer.0.output.dense.bias                        (768,)
bert.encoder.layer.0.output.LayerNorm.weight                  (768,)
bert.encoder.layer.0.output.LayerNorm.bias                    (768,)==== Output Layer ====bert.pooler.dense.weight                                  (768, 768)
bert.pooler.dense.bias                                        (768,)
classifier.weight                                           (2, 768)
classifier.bias                                                 (2,)

4.2. Optimizer & Learning Rate Scheduler

现在我们已经加载了模型，我们需要从存储的模型中获取训练超参数。

为了进行微调，作者建议从以下值中进行选择。(从论文的注释 BERT paper):

Batch size: 16, 32

Learning rate (Adam): 5e-5, 3e-5, 2e-5
Number of epochs: 2, 3, 4

作者选择的参数是：

Batch size: 32 (set when creating our DataLoaders)
Learning rate: 2e-5
Epochs: 4 (we’ll see that this is probably too many…)

参数eps = 1e-8 是"a very small number to prevent any division by zero in the implementation"(from here)

您可以在run_glue.py中找到创建AdamW优化器的方法here.

# Note: AdamW is a class from the huggingface library (as opposed to pytorch)
# AdamW是huggingface实现的类
# I believe the 'W' stands for 'Weight Decay fix"
optimizer = AdamW(model.parameters(),lr = 2e-5, # args.learning_rate - default is 5e-5, our notebook had 2e-5eps = 1e-8 # args.adam_epsilon  - default is 1e-8.)

from transformers import get_linear_schedule_with_warmup# Number of training epochs. The BERT authors recommend between 2 and 4.
# We chose to run for 4, but we'll see later that this may be over-fitting the
# training data.
epochs = 4# Total number of training steps is [number of batches] x [number of epochs].
# (Note that this is not the same as the number of training samples).
total_steps = len(train_dataloader) * epochs    # 总共4 * 241批# Create the learning rate scheduler.
scheduler = get_linear_schedule_with_warmup(optimizer, num_warmup_steps = 0, # Default value in run_glue.pynum_training_steps = total_steps)

4.3. 循环训练

下面是我们的训练循环。有很多事情要做，但从根本上来说，对于循环中的每一个过程，我们都有一个training阶段和一个validation阶段。

Thank you to Stas Bekman for contributing the insights and code for using validation loss to detect over-fitting!

Training:

打开我们的数据 inputs 和 labels
加载数据到GPU上
清除之前计算的梯度。
- 在pytorch中，除非显式清除梯度，否则梯度默认累积(对于rnn之类的东西很有用)。
Forward pass(通过网络输入数据)
Backward pass 反向传播
告诉网络使用optimizer.step()更新参数
监控进度，跟踪变量

Evalution:

同训练过程一样，打开inputs 和 labels
加载数据到GPU上
Forward pass(通过网络输入数据)
计算我们验证数据的损失，监控进度，跟踪变量

Pytorch向我们隐藏了所有详细的计算，但是我们已经对代码进行了注释，指出了每一行上发生的上述步骤。

定义一个计算精度的辅助函数。

import numpy as np# Function to calculate the accuracy of our predictions vs labels
# 这个函数来计算预测值和labels的准确度
def flat_accuracy(preds, labels):pred_flat = np.argmax(preds, axis=1).flatten()    # 取出最大值对应的索引labels_flat = labels.flatten()return np.sum(pred_flat == labels_flat) / len(labels_flat)

格式化函数时间

import time
import datetimedef format_time(elapsed):'''Takes a time in seconds and returns a string hh:mm:ss'''# Round to the nearest second.    四舍五入elapsed_rounded = int(round((elapsed)))# Format as hh:mm:ssreturn str(datetime.timedelta(seconds=elapsed_rounded))

现在开始训练，这里要修改一部分代码，作者给的代码有个地方要做修改,参考run_glue.py

import random
import numpy as np# This training code is based on the `run_glue.py` script here:
# https://github.com/huggingface/transformers/blob/5bfcd0485ece086ebcbed2d008813037968a9e58/examples/run_glue.py#L128# Set the seed value all over the place to make this reproducible. 保证可重复性
seed_val = 42random.seed(seed_val)
np.random.seed(seed_val)
torch.manual_seed(seed_val)
torch.cuda.manual_seed_all(seed_val)# We'll store a number of quantities（保存如） such as training and validation loss,
# validation accuracy, and timings.(训练loss, 验证loss, 验证准确率，训练时间)
training_stats = []# Measure the total training time for the whole run. 总训练时间
total_t0 = time.time()# For each epoch...
for epoch_i in range(0, epochs):# ========================================#               Training# ========================================# 对训练集进行一次完整的测试。print("")print('======== Epoch {:} / {:} ========'.format(epoch_i + 1, epochs))print('Training...')# Measure how long the training epoch takes.t0 = time.time()# Reset the total loss for this epoch.total_train_loss = 0# Put the model into training mode. Don't be mislead--the call to # `train` just changes the *mode*, it doesn't *perform* the training.# 这里并不是执行的训练，而是，实例化启用 BatchNormalization 和 Dropout# `dropout` and `batchnorm` layers behave differently during training# vs. test (source: https://stackoverflow.com/questions/51433378/what-does-model-train-do-in-pytorch)model.train()# For each batch of training data...for step, batch in enumerate(train_dataloader):    # 共241个batches# Progress update every 40 batches.    40步打印一次if step % 40 == 0 and not step == 0:# Calculate elapsed time in minutes.elapsed = format_time(time.time() - t0)# Report progress.print('  Batch {:>5,}  of  {:>5,}.    Elapsed: {:}.'.format(step, len(train_dataloader), elapsed))# 例: Batch    40  of    241.    Elapsed: 0:00:08.# `batch` contains three pytorch tensors:#   [0]: input ids #   [1]: attention masks#   [2]: labels # 第一步的打开数据, 第二步 将数据放到GPU `to`方法b_input_ids = batch[0].to(device)b_input_mask = batch[1].to(device)b_labels = batch[2].to(device)# 在执行 backward pass 之前，始终清除任何先前计算的梯度。# PyTorch不会自动这样做，因为累积梯度“在训练rnn时很方便”。# (source: https://stackoverflow.com/questions/48001598/why-do-we-need-to-call-zero-grad-in-pytorch)model.zero_grad()    # 第三步，梯度清零      # 执行 forward pass (在此训练批次上对模型进行评估).# The documentation for this `model` function is here: # https://huggingface.co/transformers/v2.2.0/model_doc/bert.html#transformers.BertForSequenceClassification# 它根据给定的参数和设置的标志返回不同数量的形参。# it returns the loss (because we provided labels) and the "logits"--the model outputs prior to activation.# 返回loss和"logits"--激活之前的模型输出。  model = BertForSequenceClassificationoutput = model(b_input_ids, token_type_ids=None, attention_mask=b_input_mask, labels=b_labels)# 将所有批次的训练损失累积起来，这样我们就可以在最后计算平均损失。 # `loss` 是一个单个值的tensor; the `.item()` 函数将它转为一个python numberloss, logits = output[:2]total_train_loss += loss.item()# 执行反向传播计算精度.loss.backward()# Clip the norm of the gradients to 1.0.# 梯度裁剪，防止梯度爆炸torch.nn.utils.clip_grad_norm_(model.parameters(), 1.0)# Update parameters and take a step using the computed gradient.# 更新参数，计算梯度# 优化器规定“update rule”——参数如何根据梯度、学习速率等进行修改。optimizer.step()# 更新学习率scheduler.step()# 计算平均lossavg_train_loss = total_train_loss / len(train_dataloader)            # 训练时间training_time = format_time(time.time() - t0)# 打印结果print("")print("  Average training loss: {0:.2f}".format(avg_train_loss))print("  Training epcoh took: {:}".format(training_time))# ========================================#               Validation# ========================================# 在验证集查看print("")print("Running Validation...")t0 = time.time()# 将模型置于评估模式 不使用BatchNormalization()和Dropout()model.eval()# 跟踪变量total_eval_accuracy = 0total_eval_loss = 0nb_eval_steps = 0# 在每个epoch上评估for batch in validation_dataloader:# `batch` contains three pytorch tensors:#   [0]: input ids #   [1]: attention masks#   [2]: labels b_input_ids = batch[0].to(device)b_input_mask = batch[1].to(device)b_labels = batch[2].to(device)# Tell pytorch not to bother with constructing the compute graph during# the forward pass, since this is only needed for backprop (training).with torch.no_grad():        # Forward pass, calculate logit predictions.# token_type_ids is the same as the "segment ids", which # differentiates sentence 1 and 2 in 2-sentence tasks.# The documentation for this `model` function is here: # https://huggingface.co/transformers/v2.2.0/model_doc/bert.html#transformers.BertForSequenceClassification# Get the "logits" output by the model. The "logits" are the output# values prior to applying an activation function like the softmax.(loss, logits) = model(b_input_ids, token_type_ids=None, attention_mask=b_input_mask,labels=b_labels)# 计算验证损失loss, logits = output[:2]total_eval_loss += loss.item()# Move logits and labels to CPUlogits = logits.detach().cpu().numpy()label_ids = b_labels.to('cpu').numpy()# Calculate the accuracy for this batch of test sentences, and# accumulate it over all batches.total_eval_accuracy += flat_accuracy(logits, label_ids)# 返回验证结果avg_val_accuracy = total_eval_accuracy / len(validation_dataloader)print("  Accuracy: {0:.2f}".format(avg_val_accuracy))# 计算平均复杂度avg_val_loss = total_eval_loss / len(validation_dataloader)# 时间validation_time = format_time(time.time() - t0)print("  Validation Loss: {0:.2f}".format(avg_val_loss))print("  Validation took: {:}".format(validation_time))# 记录这个epoch的所有统计数据。 方便后面可视化training_stats.append({'epoch': epoch_i + 1,'Training Loss': avg_train_loss,'Valid. Loss': avg_val_loss,'Valid. Accur.': avg_val_accuracy,'Training Time': training_time,'Validation Time': validation_time})print("")
print("Training complete!")print("Total training took {:} (h:mm:ss)".format(format_time(time.time()-total_t0)))

让我们来看看训练过程的总结。

import pandas as pd# 显示浮点数小数点后两位。
pd.set_option('precision', 2)# 从训练统计数据里，创建一个 DataFrame
df_stats = pd.DataFrame(data=training_stats)# 用'epoch'行坐标
df_stats = df_stats.set_index('epoch')# A hack to force the column headers to wrap.
#df = df.style.set_table_styles([dict(selector="th",props=[('max-width', '70px')])])# Display the table.
df_stats

Training Loss	Valid. Loss	Valid. Accur.	Training Time	Validation Time epoch
1	0.50	0.45	0.80	0:00:51
2	0.32	0.46	0.81	0:00:51
3	0.22	0.49	0.82	0:00:51
4	0.16	0.55	0.82	0:00:51

这里我跑这代码train loss没有下降，反而上升了，有了解这个问题的大大，麻烦请留言指教下

请注意，当训练损失随着时间的推移而下降时，验证损失却在增加!这表明我们训练模型的时间太长了，它对训练数据过于拟合。

(作为参考，我们使用了7,695个训练样本和856个验证样本)。

验证损失是比精度更精确的度量，因为有了精度，我们不关心确切的输出值，而只关心它落在阈值的哪一边。

如果我们预测的是正确的答案，但缺乏信心，那么验证损失将捕捉到这一点，而准确性则不会。

import matplotlib.pyplot as plt
% matplotlib inlineimport seaborn as sns# Use plot styling from seaborn.
sns.set(style='darkgrid')# Increase the plot size and font size.
sns.set(font_scale=1.5)
plt.rcParams["figure.figsize"] = (12,6)# 绘制学习曲线
plt.plot(df_stats['Training Loss'], 'b-o', label="Training")
plt.plot(df_stats['Valid. Loss'], 'g-o', label="Validation")# Label the plot.
plt.title("Training & Validation Loss")
plt.xlabel("Epoch")
plt.ylabel("Loss")
plt.legend()
plt.xticks([1, 2, 3, 4])plt.show()

5. 在测试集上的表现

现在，我们将加载holdout数据集并准备输入，就像我们对训练集所做的那样。然后，我们将使用Matthew’s correlation coefficient评估预测，因为这是更广泛的NLP社区用于评估CoLA性能的指标。在这个指标下，+1是最好的分数，-1是最差的分数。通过这种方式，我们可以看到针对这个特定任务的先进模型的性能如何。

5.1. 数据准备

我们需要应用与训练数据相同的所有步骤来准备测试数据集。

import pandas as pd# 加载数据
df = pd.read_csv("./cola_public/raw/out_of_domain_dev.tsv", delimiter='\t', header=None, names=['sentence_source', 'label', 'label_notes', 'sentence'])# 显示句子数量
print('Number of test sentences: {:,}\n'.format(df.shape[0]))# 创建句子和标签列表
sentences = df.sentence.values
labels = df.label.values# Tokenize
input_ids = []
attention_masks = []# For every sentence...
for sent in sentences:# `encode_plus` will:#   (1) Tokenize the sentence.#   (2) 添加 `[CLS]` token 到开始#   (3) 添加 `[SEP]` token 到结束#   (4) 映射tokens 到 IDs.#   (5) 填充或截断句子到`max_length`#   (6) Create attention masks for [PAD] tokens.encoded_dict = tokenizer.encode_plus(sent,                      # 对句子做encode.add_special_tokens = True, # Add '[CLS]' and '[SEP]'max_length = 64,           # Pad & truncate all sentences.pad_to_max_length = True,return_attention_mask = True,   # Construct attn. masks.return_tensors = 'pt',     # Return pytorch tensors.)# 将已编码的句子添加到列表中。  input_ids.append(encoded_dict['input_ids'])# 以及它的注意力掩码(简单地区分填充和非填充)。attention_masks.append(encoded_dict['attention_mask'])# Convert the lists into tensors.
input_ids = torch.cat(input_ids, dim=0)
attention_masks = torch.cat(attention_masks, dim=0)
labels = torch.tensor(labels)# Set the batch size.
batch_size = 32  # Create the DataLoader.
prediction_data = TensorDataset(input_ids, attention_masks, labels)
prediction_sampler = SequentialSampler(prediction_data)
prediction_dataloader = DataLoader(prediction_data, sampler=prediction_sampler, batch_size=batch_size)

Number of test sentences: 516

5.2. 在测试集上评估

准备好测试集之后，我们可以应用我们的微调模型来生成测试集的预测。

# Prediction on test setprint('Predicting labels for {:,} test sentences...'.format(len(input_ids)))# 在测试模型
model.eval()# 跟踪变量
predictions , true_labels = [], []# Predict
for batch in prediction_dataloader:# Add batch to GPUbatch = tuple(t.to(device) for t in batch)# Unpack the inputs from our dataloaderb_input_ids, b_input_mask, b_labels = batch# 不让模型计算或存储梯度，节省内存和加速预测with torch.no_grad():# Forward pass, calculate logit predictionsoutputs = model(b_input_ids, token_type_ids=None, attention_mask=b_input_mask)logits = outputs[0]# Move logits and labels to CPUlogits = logits.detach().cpu().numpy()label_ids = b_labels.to('cpu').numpy()# Store predictions and true labelspredictions.append(logits)true_labels.append(label_ids)print('    DONE.')

CoLA基准的精度是用“Matthews correlation coefficient”来测量的。(MCC)。

我们在这里使用MCC是因为类是不平衡的:

print('Positive samples: %d of %d (%.2f%%)' % (df.label.sum(), len(df.label), (df.label.sum() / len(df.label) * 100.0)))

Positive samples: 354 of 516 (68.60%)

# 计算相关系数
from sklearn.metrics import matthews_corrcoefmatthews_set = []# 使用Matthew相关系数对每个测试批进行评估
print('Calculating Matthews Corr. Coef. for each batch...')# For each input batch...
for i in range(len(true_labels)):# 这个批处理的预测是一个2列的ndarray(一个列是“0”，一个列是“1”)。 # 选择值最高的label，并将其转换为0和1的列表。pred_labels_i = np.argmax(predictions[i], axis=1).flatten()# Calculate and store the coef for this batch.  matthews = matthews_corrcoef(true_labels[i], pred_labels_i)                matthews_set.append(matthews)

最终的分数将基于整个测试集，但是让我们看一下单个批次的分数，以了解批次之间度量的可变性。

每批有32个句子，除了最后一批只有(516% 32)= 4个测试句子。

创建一个柱状图，显示每批测试样品的MCC分数。

ax = sns.barplot(x=list(range(len(matthews_set))), y=matthews_set, ci=None)plt.title('MCC Score per Batch')
plt.ylabel('MCC Score (-1 to +1)')
plt.xlabel('Batch #')plt.show()

现在我们将合并所有批次的结果并计算我们最终的MCC分数。

# 合并所有批次的结果。
flat_predictions = np.concatenate(predictions, axis=0)# 对于每个样本，选择得分较高的标签(0或1)。
flat_predictions = np.argmax(flat_predictions, axis=1).flatten()# 将每个批次的正确标签组合成一个单独的列表。
flat_true_labels = np.concatenate(true_labels, axis=0)# Calculate the MCC
mcc = matthews_corrcoef(flat_true_labels, flat_predictions)print('Total MCC: %.3f' % mcc)

在大约半个小时的时间里，我们没有做任何超参数的调整(learning rate, epochs, batch size, ADAM properties属性等)，我们就获得了一个很好的分数。

为了使分数最大化，我们应该删除“验证集”(我们用来帮助确定要训练多少个纪元)，并在整个训练集上训练。

库将基准测试此处的预期精度文档为“49.23”。

官方排行 here.

请注意(由于数据集的大小较小?)在不同的运行中，精度可能会有很大的变化。

总结

这篇文章演示了使用预先训练好的BERT模型，不管你感兴趣的是什么特定的NLP任务，你都可以使用pytorch接口，用最少的努力和训练时间，快速有效地创建一个高质量的模型。

附录

A1. Saving & Loading Fine-Tuned Model

(取自’ run_glue。py 'here)将模型和标记器写入磁盘。

import os# 保存best-practices:如果您使用模型的默认名称，您可以使用from_pretraining()重新加载它
# Saving best-practices: if you use defaults names for the model, you can reload it using from_pretrained()output_dir = './model_save/'# 如果需要，创建输出目录
if not os.path.exists(output_dir):os.makedirs(output_dir)print("Saving model to %s" % output_dir)# 使用`save_pretrained()`保存训练过的模型、配置和标记器。
# 用`from_pretrained()`重新加载模型。
model_to_save = model.module if hasattr(model, 'module') else model  # 注意distributed/parallel training
model_to_save.save_pretrained(output_dir)
tokenizer.save_pretrained(output_dir)# Good practice: 保存训练好的模型于模型参数
# torch.save(args, os.path.join(output_dir, 'training_args.bin'))

Revision History

Version 3 - Mar 18th, 2020 - (current)

Simplified the tokenization and input formatting (for both training and test) by leveraging the tokenizer.encode_plus function.
encode_plus handles padding and creates the attention masks for us.
Improved explanation of attention masks.
Switched to using torch.utils.data.random_split for creating the training-validation split.
Added a summary table of the training statistics (validation loss, time per epoch, etc.).
Added validation loss to the learning curve plot, so we can see if we’re overfitting.
- Thank you to Stas Bekman for contributing this!
Displayed the per-batch MCC as a bar plot.

Version 2 - Dec 20th, 2019 - link

huggingface renamed their library to transformers.
Updated the notebook to use the transformers library.

Version 1 - July 22nd, 2019

Initial version.

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