code: def shared_step(
      self, batch, batch_idx
  ) -> Tuple[Tensor, Dict[int, Tensor]]:
      features, targets = batch[0], batch[1]
      predictions = self.forward(features)
      loss = self.criterion(predictions, targets)
      _, predicted_classes = predictions.topk(max(self.topk))
      topk = mean_topk_accuracy(predicted_classes, targets, k=self.topk)
      return loss, topk
  
file_path: lightly/lightly/utils/benchmarking/online_linear_classifier.py
imports: from typing import Dict, Tuple
  from pytorch_lightning import LightningModule
  from torch import Tensor
  from torch.nn import CrossEntropyLoss, Linear
  from lightly.utils.benchmarking.topk import mean_topk_accuracy
label_desc: Use image node_id 3 for calling the OnlineLinearClassifier obj's underlying member method code with example usage: obj.shared_step(batch, batch_idx) and returns: loss,  topk