code: def on_train_start(self, state: State, unit: TTrainUnit) -> None:
      self.called_hooks.add("on_train_start")
  
imports: import unittest
  from typing import Set, Union
  from unittest.mock import MagicMock
  from torchtnt.framework._callback_handler import _get_implemented_callback_mapping, CallbackHandler
  from torchtnt.framework.callback import Callback
  from torchtnt.framework.callbacks.lambda_callback import Lambda
  from torchtnt.framework.state import EntryPoint, State
  from torchtnt.framework.unit import TEvalUnit, TPredictUnit, TTrainUnit
  from torchtnt.utils.timer import Timer
label_desc: Use image node_id 3 for calling the DummyCallback obj's underlying member method code with example usage: obj.on_train_start(state, unit) without return types