code: def test_before_val_epoch(self):
      runner = deepcopy(self.runner)
  
      # Test when dataset.metainfo['classes'] is None
      runner.val_dataloader.dataset = self.none_classmeta_dataset
      self.hook.before_val_epoch(runner)
      runner.logger.warning.assert_called_once()
      # Test when dataset.metainfo['classes'] is a str
      runner.val_dataloader.dataset = self.str_classmeta_dataset
      with self.assertRaises(AssertionError):
          self.hook.before_val_epoch(runner)
  
      runner.val_dataloader.dataset = self.normal_classmeta_dataset
      # Test `num_classes` of model is compatible with dataset
      runner.model = self.valid_model
      self.hook.before_val_epoch(runner)
      # Test `num_classes` of model is not compatible with dataset
      runner.model = self.invalid_model
      with self.assertRaises(AssertionError):
          self.hook.before_val_epoch(runner)
  
file_path: mmdetection/tests/test_engine/test_hooks/test_num_class_check_hook.py
imports: from copy import deepcopy
  from unittest import TestCase
  from unittest.mock import Mock
  from mmcv.cnn import VGG
  from mmengine.dataset import BaseDataset
  from torch import nn
  from mmdet.engine.hooks import NumClassCheckHook
  from mmdet.models.roi_heads.mask_heads import FusedSemanticHead
label_desc: Use image node_id 3 for calling the TestNumClassCheckHook obj's underlying member method code with example usage: obj.test_before_val_epoch() without return types