class_docstr: Class for chaining callbacks.
  
  :param callbacks: (List[BaseCallback]) A list of callbacks that will be called
      sequentially.
class_docstr_tok: ["Class","for","chaining","callbacks",".",":","param","callbacks",":","(","List","[","BaseCallback","]",")","A","list","of","callbacks","that","will","be","called","sequentially","."]
code: def _on_rollout_start(self) -> None:
      for callback in self.callbacks:
          callback.on_rollout_start()
  
imports: import os
  from abc import ABC
  import warnings
  import typing
  from typing import Union, List, Dict, Any, Optional
  import gym
  import numpy
  from stable_baselines.common.vec_env import VecEnv, sync_envs_normalization, DummyVecEnv
  from stable_baselines.common.evaluation import evaluate_policy
  from stable_baselines import logger
label_desc: Use image node_id 4 for calling the CallbackList obj's underlying member method code with example usage: obj._on_rollout_start() without return types