def build_evaluator(cls, cfg, dataset_name):
        """
        Returns:
            DatasetEvaluator or None

        It is not implemented by default.
        """
        raise NotImplementedError(
            """
If you want DefaultTrainer to automatically run evaluation,
please implement `build_evaluator()` in subclasses (see train_net.py for example).
Alternatively, you can call evaluation functions yourself (see Colab balloon tutorial for example).
"""
        )

        Returns:
            DatasetEvaluator or None

        It is not implemented by default.
        
video_chat_with_MOSS/models/grit_src/third_party/CenterNet2/detectron2/engine/defaults.py