code: def training_step(self, batch, batch_idx):
      input, target = batch
      output = self.model(input, target)
      loss = F.nll_loss(output, target.view(-1))
      self.log("train_loss", loss, prog_bar=True)
      return loss
  
file_path: pytorch-lightning/examples/pytorch/basics/transformer.py
imports: import lightning
  import torch
  import torch.nn.functional
  from lightning.pytorch.demos import Transformer, WikiText2
  from torch.utils.data import DataLoader, random_split
label_desc: Use image node_id 2 for calling the LanguageModel obj's underlying member method code with example usage: obj.training_step(batch, batch_idx) and returns: loss