I have a simple NER model that uses a word2vec embeddings layer followed by a linear layer and a softmax function.
To compute the loss after the softmax function, I use the BCELoss function.
class SemanticCRF(nn.Module):

    def __init__(self):
        super(SemanticCRF, self).__init__()

        # log reg
        self.hidden2tag = nn.Linear(300, 2)

        # crf
        self.crf_layer = CRF(2, batch_first=True)

        # binary cross entropy loss
        self.criterion = nn.BCELoss()


    def forward(self, input_ids=None, attention_mask=None, labels=None):

        # log reg
        probablities_ = F.softmax ( self.hidden2tag( input_ids.float() ) )
        probablities__ = torch.argmax(probablities_, dim=2)
        print(probablities__)
        
        probablities___ = torch.masked_select(probablities__, attention_mask)
        labels = torch.masked_select(labels, attention_mask)

        print('Logits: ', probablities__.dtype)
        print('Labels: ', labels.dtype)

        loss = self.criterion(probablities___.float() , labels.float())
        # print(loss)
        print(loss.requires_grad) 
        # return loss, torch.Tensor(probablities.float()), labels.float()
        return loss, probablities___, labels


However, this causes the following error. The model worked perfectly fine with a CRF layer that output loss which was backpropagated without error. The error comes up only with BCELoss. I am using BCELoss because there are only two entities in my NER task. With BCELoss, loss.requires_grad is set to False.
Traceback (most recent call last):
  File "TrainModel.py", line 264, in <module>
    train(model, optimizer, scheduler, train_dataloader, development_dataloader, args)
  File "TrainModel.py", line 128, in train
    abs(b_loss).backward()
  File "/home//lib/python3.6/site-packages/torch/tensor.py", line 166, in backward
    torch.autograd.backward(self, gradient, retain_graph, create_graph)
  File "/home//lib/python3.6/site-packages/torch/autograd/__init__.py", line 99, in backward
    allow_unreachable=True)  # allow_unreachable flag
RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn