code: def training_step(self, batch, batch_idx):
      x, y = batch
      y_hat = self(x)
      return F.cross_entropy(y_hat, y)
  
file_path: pytorch-lightning/examples/pytorch/ipu/mnist_sample.py
imports: import torch
  from lightning.pytorch import LightningModule, Trainer
  from lightning.pytorch.demos.mnist_datamodule import MNISTDataModule
  from torch.nn import functional
label_desc: Use image node_id 3 for calling the LitClassifier obj's underlying member method code with example usage: obj.training_step(batch, batch_idx) and returns: F