code: def validation_step(self, batch, batch_idx):
      x, y = batch
      logits = self(x)
      loss = F.nll_loss(logits, y)
      preds = torch.argmax(logits, dim=1)
      acc = multiclass_accuracy(preds, y, num_classes=self.num_classes)
      self.log("val_acc", acc, prog_bar=False)
      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 4 for calling the MnistModel obj's underlying member method code with example usage: obj.validation_step(batch, batch_idx) and returns: loss