code: def training_step(self, batch, batch_nb):
      x, y = batch
      loss = F.cross_entropy(self(x), y)
      self.log("loss", loss)
      return loss
  
file_path: labml/samples/lightning/mnist.py
imports: import pytorch_lightning
  import torch
  from torch.nn import functional
  from torch.utils.data import DataLoader
  from torchvision import transforms
  from torchvision.datasets import MNIST
  from labml import lab, experiment
  from labml.utils.lightning import LabMLLightningLogger
label_desc: Use image node_id 3 for calling the MNISTModel obj's underlying member method code with example usage: obj.training_step(batch, batch_nb) and returns: loss