code: def test_returns_correct_instances(self):
      rb, observer = replay_buffer_utils.get_reverb_buffer_and_observer(
          self._get_mock_spec(), sequence_length=1, port=None
      )
      self.assertIsInstance(rb, reverb_replay_buffer.ReverbReplayBuffer)
      self.assertIsInstance(
          observer, reverb_utils.ReverbAddTrajectoryObserver
      )
      rb.local_server.stop()
  
file_path: agents/tf_agents/train/utils/replay_buffer_utils_test.py
imports: import tensorflow
  from tf_agents import specs
  from tf_agents.replay_buffers import reverb_replay_buffer
  from tf_agents.replay_buffers import reverb_utils
  from tf_agents.train.utils import replay_buffer_utils
  from tf_agents.utils import lazy_loader
  from tf_agents.utils import test_utils
label_desc: Use image node_id 2 for calling the ReplayBufferUtilsTest obj's underlying member method code with example usage: obj.test_returns_correct_instances() without return types