[Bert] 提取特征之后训练模型报梯度图错误

报错：
RuntimeError: stack(): functions with out=… arguments don’t support automatic differentiation, but one of the arguments requires grad.

或者

RuntimeError: Trying to backward through the graph a second time (or directly access saved tensors after they have already been freed). Saved intermediate values of the graph are freed when you call .backward() or autograd.grad(). Specify retain_graph=True if you need to backward through the graph a second time or if you need to access saved tensors after calling backward.

原因：

训练模型的时候，输入数据x，y不应该requires_grad，而bert模型输出的embeddings默认是requires_grad的，所以会报错。

解决方法：

提取完embeddings之后，使用 embeddings.detach() 解除绑定就行了。

最后的代码：

from transformers import BertTokenizer, BertModel

class BertFeatureExtractor:
    def __init__(self):
        self.tokenizer = BertTokenizer.from_pretrained('bert-base-chinese')
        self.model = BertModel.from_pretrained('bert-base-chinese')

    def extract_features(self, text: str):
        inputs = self.tokenizer(text, return_tensors="pt")
        if len(inputs["input_ids"]) > 512:
            inputs["input_ids"] = inputs["input_ids"][:512]
            inputs["attention_mask"] = inputs["attention_mask"][:512]
        outputs = self.model(**inputs)
        return outputs.last_hidden_state[:,0,:]
    
feat = feat.detach()