code: def training_step(self, batch, batch_idx):
      x, y = batch
      y_hat = self(x)
      loss = F.cross_entropy(y_hat, y)
      return {"loss": loss}
  
file_path: fastai/nbs/examples/migrating_lightning.py
imports: from fastai.data.load import DataLoader
  import os, torch
  from torch.nn import functional
  from torchvision.datasets import MNIST
  from torchvision import transforms
  from lightning import LightningModule
label_desc: Use image node_id 3 for calling the LitModel obj's underlying member method code with example usage: obj.training_step(batch, batch_idx) and returns: dict