code: def test_custom_callback(self):
      # TF 1.x is not supported
      if utils.is_tf_v1():
          return
  
      # Setting up custom callback with mock calls
      _MockCallable.reset()
  
      self._fit_kwargs["callbacks"] = [CustomCallbackExample()]
      client._serialize_assets(
          self._remote_dir, self._model, **self._fit_kwargs
      )
  
      # Verify callback function has not been called yet.
      _MockCallable.mock_callable.assert_not_called()
  
      remote.run(self._remote_dir, MIRRORED_STRATEGY_NAME)
      # Verifying callback functions triggered properly
      _MockCallable.mock_callable.assert_called_once_with()
  
imports: import json
  import os
  import tempfile
  from unittest import mock
  from absl import flags
  import numpy
  import tensorflow
  from tensorflow_cloud.tuner import cloud_fit_client
  from tensorflow_cloud.tuner import cloud_fit_remote
  from tensorflow_cloud.tuner import cloud_fit_utils
label_desc: Use image node_id 4 for calling the CloudFitRemoteTest obj's underlying member method code with example usage: obj.test_custom_callback() without return types