code: def loss_fn(self, input, target, mask):
      y_pred = self._get_lstm_feature(input)
      return -self.crf.forward(y_pred, target, mask)
  
file_path: DeepKE/src/deepke/name_entity_re/standard/models/BiLSTM_CRF.py
imports: import torch.nn
  from torchcrf import CRF
label_desc: Use image node_id 4 for calling the BiLSTM_CRF obj's underlying member method code with example usage: obj.loss_fn(input, target, mask) and returns: unknown