class_docstr: Check whether the `num_classes` in head matches the length of `classes`
  in `dataset.metainfo`.
docstr: Check whether the training dataset is compatible with head.
  
  Args:
      runner (:obj:`Runner`): The runner of the training or evaluation
          process.
code: def before_train_epoch(self, runner: Runner) -> None:
      """Check whether the training dataset is compatible with head.
  
      Args:
          runner (:obj:`Runner`): The runner of the training or evaluation
              process.
      """
      self._check_head(runner, "train")
  
file_path: mmdetection/mmdet/engine/hooks/num_class_check_hook.py
imports: from mmcv.cnn import VGG
  from mmengine.hooks import Hook
  from mmengine.runner import Runner
  from mmdet.registry import HOOKS
label_desc: Use image node_id 2 for calling the NumClassCheckHook obj's underlying member method code with example usage: obj.before_train_epoch(runner) without return types