class_docstr: Used to chain a list of callbacks to one Callback. Boolean hook methods are
  logically joined with AND, meaning that if at least one callback method
  returns False, the training is stopped.
  
  Attributes
  ----------
  callbacks
      A list of gluonts.mx.trainer.callback.Callback's.
code: def _exec(
      self, function_name: str, *args: Any, **kwargs: Any
  ) -> Union[List[bool], List[None]]:
      return [
          getattr(callback, function_name)(*args, **kwargs)
          for callback in self.callbacks
      ]
  
imports: from dataclasses import dataclass, field
  from typing import List, Union, Dict, Any, Optional
  import logging
  import math
  import time
  import mxnet.gluon.nn
  import mxnet
  from mxnet import gluon
  from gluonts.core.component import validated
  from gluonts.mx.util import copy_parameters
  from gluonts.pydantic import BaseModel, PrivateAttr
label_desc: Use image node_id 2 for calling the CallbackList obj's underlying member method code with example usage: obj._exec(function_name) and returns: unknown