class_docstr: Wrapper class for a list of callbacks.
code: def on_train_start(self, trainer: gluon.Trainer) -> None:
      for callback in self.callbacks:
          callback.on_train_start(trainer)
  
file_path: gluonts/src/gluonts/nursery/tsbench/src/tsbench/gluonts/callbacks/base.py
imports: from typing import List
  from mxnet import gluon
  from mxnet.gluon import nn
label_desc: Use image node_id 2 for calling the CallbackList obj's underlying member method code with example usage: obj.on_train_start(trainer) without return types