code: def training_step(self, batch, batch_idx):
      x, y = batch
      logits = self(x)
      loss = F.nll_loss(logits, y)
      return loss
  
imports: import argparse
  import logging
  import os
  import time
  import warnings
  import torch
  from pytorch_lightning import LightningModule, loggers, Trainer
  from torch import nn
  from torch.nn import functional
  from torch.utils.data import DataLoader
  from torchmetrics.functional.classification.accuracy import multiclass_accuracy
  from torchvision import transforms
  from torchvision.datasets import MNIST
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_idx) and returns: loss