code: def training_step(self, batch, batch_idx):
      X, y = batch
      y_hat = self.backbone(X)
      loss = F.cross_entropy(y_hat, y)
      self.log("train_loss", loss, on_epoch=True)
      return loss
  
file_path: underthesea/examples/ner/train.py
imports: import torch
  import pytorch_lightning
  from torch.nn import functional
  import torch.nn
label_desc: Use image node_id 3 for calling the SequenceTokenClassifier obj's underlying member method code with example usage: obj.training_step(batch, batch_idx) and returns: loss