code: def validation_step(self, batch, batch_idx) -> Tensor:
      images, targets = batch[0], batch[1]
      features = self.linear(images)
      cls_loss, cls_log = self.online_classifier.validation_step(
          (features, targets), batch_idx
      )
      self.log_dict(cls_log)
      return cls_loss
  
file_path: lightly/tests/utils/benchmarking/test_online_linear_classifier.py
imports: import pytest
  import torch
  from pytorch_lightning import LightningModule, Trainer
  from torch import Tensor, nn
  from torch.optim import SGD
  from torch.utils.data import DataLoader
  from torchvision.datasets import FakeData
  from torchvision.transforms import ToTensor
  from lightly.utils.benchmarking import OnlineLinearClassifier
label_desc: Use image node_id 3 for calling the _DummyModule obj's underlying member method code with example usage: obj.validation_step(batch, batch_idx) and returns: cls_loss