code: def training_step(self, batch, batch_idx):
      x, y = batch
      logits = self(x)
      loss = F.nll_loss(logits, y)
      self.log(
          "my_loss",
          loss,
          on_step=True,
          on_epoch=True,
          prog_bar=True,
          logger=True,
      )
      return loss
  
file_path: guildai/examples/pytorch-lightning/mnist.py
imports: import os
  import argparse
  import torch
  from torch import nn
  from torch.nn import functional
  from torch.optim import Adam
  from torch.utils.data import DataLoader, random_split
  from torchvision import datasets, transforms
  from torchvision.datasets import MNIST
  from pytorch_lightning import Trainer
  from pytorch_lightning.core.lightning import LightningModule
label_desc: Use image node_id 3 for calling the LitMNIST obj's underlying member method code with example usage: obj.training_step(batch, batch_idx) and returns: loss