【Transformers基础入门篇4】基础组件之Model

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本文为 https://space.bilibili.com/21060026/channel/collectiondetail?sid=1357748的视频学习笔记

项目地址为：https://github.com/zyds/transformers-code

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一、Model简介

1.1 Transformer

既然这个包的名字叫Transformers，那么大部分整个模型都是基于Transformer架构。

• 原始的Tranformer为编码器(Encoder)、解码器（Decoder)模型
• Encoder部分接收输入并构建其完整的特征表示，Decoder部分使用Encoder的编码结果以及其他的输入生成目标序列
• 无论是编码器还是解码器，均由多个TransformerBlock堆叠而成
• TransformerBloc由注意力机制（Attention）和FFN组成。

1.2 注意力机制

• 注意力机制的使用是Transformer的一个核心特征，在计算当前次的特征表示时，可以通过注意力机制有选择性的告诉模型使用哪些上下文
  

1.3 模型类型

• 编码器模型：自编码模型，使用Encoder，拥有双向的注意力机制，即计算每一个词的特征都看到完整的上下文
• 解码器模型：自回归模型，使用Decoder，拥有单向的注意力机制，即计算每一个词的特征时，都只能看到上文，无法看到下文
• 编码器解码器模型：序列到序列模型，使用Encoder+Decoder，Encoder部分拥有双向的注意力，Decoder部分使用单向注意力
  

二、Model Head

2.1 什么是 Model Head

• Model Head是连接在模型后的层，通常为1个或多个全连接层
• Model Head将模型的编码的表示结果进行映射，以解决不同类型的任务
  

2.2 Transformers中的Model Head

• *Model: 模型本身，只返回编码结果
• *ForCausalLM: 纯的解码器模型
• *ForMaskedLM：像Bert那种，在句子中随机Mask一些token，然后来预测token是什么
• *ForSeq2SeqLM： 序列到序列的模型
• *ForMultipleChoice：多项选择任务
• *ForQuestionAnswering
• *ForSequenceClassification
• *ForTokenClassification
• …其他

下图是Bert刚提出来时，给出的不同任务的解决方案

• 句子对分类：取CLS 分类
• 单一句子分类 取CLS 分类
• 问答： 将Question和Paragraph同时输入，然后对Paragraph做两个全连接，记住起始和结束位置。
• NER任务（命名实体识别）：对每一个token进行分类
  

三、Model基本使用方法

3.0 模型下载-浏览器下载

https://huggingface.co/hfl/rbt3/tree/main
自行下，然后放到hfl/rbt3文件夹

也可以通过代码下载，在huggingface模型上选

!git clone "https://huggingface.co/hfl/rbt3"  # 会下载所有模型，包括tf、flax模型
!git lfs clone "https://huggingface.co/hfl/rbt3" --include="*.bin" # 仅下载pytorch模型

3.1 模型加载与保存

在HuggingFace上选一个小模型 hfl/rbt3

from transformers import AutoConfig, AutoModel, AutoTokenizer
model = AutoModel.from_pretrained("../../models/hfl/rbt3")

3.2 配置加载参数

model.config

3.3 加载config文件

# 加载config文件
config = AutoConfig.from_pretrained("../../models/hfl/rbt3")
config

输出BertConfig，那么我们就可以进入BertConfig来看具体的参数
from transformers import BertConfig
查看BertConfig

class BertConfig(PretrainedConfig):r"""This is the configuration class to store the configuration of a [`BertModel`] or a [`TFBertModel`]. It is used toinstantiate a BERT model according to the specified arguments, defining the model architecture. Instantiating aconfiguration with the defaults will yield a similar configuration to that of the BERT[google-bert/bert-base-uncased](https://huggingface.co/google-bert/bert-base-uncased) architecture.Configuration objects inherit from [`PretrainedConfig`] and can be used to control the model outputs. Read thedocumentation from [`PretrainedConfig`] for more information.Args:vocab_size (`int`, *optional*, defaults to 30522):Vocabulary size of the BERT model. Defines the number of different tokens that can be represented by the`inputs_ids` passed when calling [`BertModel`] or [`TFBertModel`].hidden_size (`int`, *optional*, defaults to 768):Dimensionality of the encoder layers and the pooler layer.num_hidden_layers (`int`, *optional*, defaults to 12):Number of hidden layers in the Transformer encoder.num_attention_heads (`int`, *optional*, defaults to 12):Number of attention heads for each attention layer in the Transformer encoder.intermediate_size (`int`, *optional*, defaults to 3072):Dimensionality of the "intermediate" (often named feed-forward) layer in the Transformer encoder.hidden_act (`str` or `Callable`, *optional*, defaults to `"gelu"`):The non-linear activation function (function or string) in the encoder and pooler. If string, `"gelu"`,`"relu"`, `"silu"` and `"gelu_new"` are supported.hidden_dropout_prob (`float`, *optional*, defaults to 0.1):The dropout probability for all fully connected layers in the embeddings, encoder, and pooler.attention_probs_dropout_prob (`float`, *optional*, defaults to 0.1):The dropout ratio for the attention probabilities.max_position_embeddings (`int`, *optional*, defaults to 512):The maximum sequence length that this model might ever be used with. Typically set this to something largejust in case (e.g., 512 or 1024 or 2048).type_vocab_size (`int`, *optional*, defaults to 2):The vocabulary size of the `token_type_ids` passed when calling [`BertModel`] or [`TFBertModel`].initializer_range (`float`, *optional*, defaults to 0.02):The standard deviation of the truncated_normal_initializer for initializing all weight matrices.layer_norm_eps (`float`, *optional*, defaults to 1e-12):The epsilon used by the layer normalization layers.position_embedding_type (`str`, *optional*, defaults to `"absolute"`):Type of position embedding. Choose one of `"absolute"`, `"relative_key"`, `"relative_key_query"`. Forpositional embeddings use `"absolute"`. For more information on `"relative_key"`, please refer to[Self-Attention with Relative Position Representations (Shaw et al.)](https://arxiv.org/abs/1803.02155).For more information on `"relative_key_query"`, please refer to *Method 4* in [Improve Transformer Modelswith Better Relative Position Embeddings (Huang et al.)](https://arxiv.org/abs/2009.13658).is_decoder (`bool`, *optional*, defaults to `False`):Whether the model is used as a decoder or not. If `False`, the model is used as an encoder.use_cache (`bool`, *optional*, defaults to `True`):Whether or not the model should return the last key/values attentions (not used by all models). Onlyrelevant if `config.is_decoder=True`.classifier_dropout (`float`, *optional*):The dropout ratio for the classification head.Examples:```python>>> from transformers import BertConfig, BertModel>>> # Initializing a BERT google-bert/bert-base-uncased style configuration>>> configuration = BertConfig()>>> # Initializing a model (with random weights) from the google-bert/bert-base-uncased style configuration>>> model = BertModel(configuration)>>> # Accessing the model configuration>>> configuration = model.config```"""

3.2 模型调用

3.2.1 带ModelHead的调用

sen = "弱小的我也有大梦想！"
tokenizer = AutoTokenizer.from_pretrained("../../models/hfl/rbt3")
inputs = tokenizer(sen, return_tensors="pt")
output = model(**inputs)

3.2.2 不带ModelHead的调用

model = AutoModel.from_pretrained("../../models/hfl/rbt3", output_attentions=True)
output = model(**inputs)
output.last_hidden_state.size() # 输出序列的输入编码的维度
len(inputs["input_ids"][0]) # 查看输入ID的长度

输出的12就是字的长度

3.2.3带ModelHead的调用

from transformers import AutoModelForSequenceClassification, BertForSequenceClassification
clz_model = AutoModelForSequenceClassification.from_pretrained("../../models/hfl/rbt3")
clz_model(**inputs)

输出可以看到任务是一个文本分类任务，输出loss，logits .
这时候logits还只有一个二分类模型，我们可以调整成10类

clz_model = AutoModelForSequenceClassification.from_pretrained("../../models/hfl/rbt3", num_labels=10)
clz_model(**inputs)

那么怎么看num_labels参数呢？
定位到 BertForSequenceClassification类实现里看到 config里有num_labels

class BertForSequenceClassification(BertPreTrainedModel):def __init__(self, config):super().__init__(config)self.num_labels = config.num_labelsself.config = configself.bert = BertModel(config) classifier_dropout = (config.classifier_dropout if config.classifier_dropout is not None else config.hidden_dropout_prob)self.dropout = nn.Dropout(classifier_dropout)self.classifier = nn.Linear(config.hidden_size, config.num_labels)# Initialize weights and apply final processingself.post_init()

四、模型微调代码实例

• 任务类型： 文本分类

• 使用模型： hfl/rbt3

• 数据集地址 https://github.com/SophonPlus/ChineseNlpCorpus

• Step1 导入相关包

from transformers import AutoTokenizer, AutoModelForSequenceClassification,BertForSequenceClassification

• Step2 加载数据

import pandas as pd
data = pd.read_csv("./ChnSentiCorp_htl_all.csv")
data = data.dropna() # 清除数据损坏的部分

• Step3 创建Dataset

class MyDataset(Dataset):def __init__(self) -> None:super().__init__()self.data = pd.read_csv("./ChnSentiCorp_htl_all.csv")self.data = self.data.dropna()def __getitem__(self, index):return self.data.iloc[index]["review"], self.data.iloc[index]["label"]def __len__(self):return len(self.data)

dataset = MyDataset()
for i in range(5):print(dataset[i])

• Step4 划分数据集

rom torch.utils.data import random_split
trainset, validset = random_split(dataset, lengths=[0.9, 0.1])
len(trainset), len(validset)

• Step5 创建Dataloader

这里读取数据，然后写处理函数传到Dataloder中collate_fn，可以实现批量处理

import torchtokenizer = AutoTokenizer.from_pretrained("../../models/hfl/rbt3")def collate_func(batch):texts, labels = [], []for item in batch:texts.append(item[0])labels.append(item[1])# truncation=True 过长做截断inputs = tokenizer(texts, max_length=128, padding="max_length", truncation=True, return_tensors="pt")inputs["labels"] = torch.tensor(labels)return inputs

from torch.utils.data import DataLoadertrainloader = DataLoader(trainset, batch_size=32, shuffle=True, collate_fn=collate_func)
validloader = DataLoader(validset, batch_size=64, shuffle=False, collate_fn=collate_func)# 查看loader
next(enumerate(validloader))[1]

• step6 创建模型以及优化器

from torch.optim import Adammodel = AutoModelForSequenceClassification.from_pretrained("../../models/hfl/rbt3")if torch.cuda.is_available():model = model.cuda()
optimizer = Adam(model.parameters(), lr=2e-5)

• step7 训练与验证

def evaluate():model.eval()acc_num = 0with torch.inference_mode():for batch in validloader:if torch.cuda.is_available():batch = {k: v.cuda() for k, v in batch.items()}output = model(**batch)pred = torch.argmax(output.logits, dim=-1)acc_num += (pred.long() == batch["labels"].long()).float().sum()return acc_num / len(validset)def train(epoch=3, log_step=100):global_step = 0for ep in range(epoch):model.train()for batch in trainloader:if torch.cuda.is_available():batch = {k: v.cuda() for k, v in batch.items()}optimizer.zero_grad()output = model(**batch)output.loss.backward()optimizer.step()if global_step % log_step == 0:print(f"ep: {ep}, global_step: {global_step}, loss: {output.loss.item()}")global_step += 1acc = evaluate()print(f"ep: {ep}, acc: {acc}")

• step9 模型训练与预测

train()
# sen = "我觉得这家酒店不错，饭很好吃！"
sen = "我觉得这家酒店太差了，饭很难吃！"
id2_label = {0: "差评！", 1: "好评！"}
model.eval()
with torch.inference_mode():inputs = tokenizer(sen, return_tensors="pt")inputs = {k: v.cuda() for k, v in inputs.items()}logits = model(**inputs).logitspred = torch.argmax(logits, dim=-1)print(f"输入：{sen}\n模型预测结果:{id2_label.get(pred.item())}")

尝试pipeline

from transformers import pipelinemodel.config.id2label = id2_label
pipe = pipeline("text-classification", model=model, tokenizer=tokenizer, device=0)
pipe(sen)

输出如下：