code: def tearDownClass(cls):
      ray.shutdown()
  
imports: from collections import Counter
  import unittest
  import ray
  from ray.rllib.algorithms.callbacks import DefaultCallbacks
  from ray.rllib.algorithms.ppo import PPOConfig
  from ray.rllib.env.single_agent_env_runner import SingleAgentEnvRunner
  from ray.rllib.utils.test_utils import framework_iterator
label_desc: Use image node_id 2 for calling the TestCallbacks obj's underlying member method code with example usage: obj.tearDownClass(cls) without return types