code: def on_shutdown(
      self, worker_group: WorkerGroup, backend_config: TestConfig
  ):
      pass
  
imports: import math
  import os
  import tempfile
  import time
  from unittest.mock import patch
  import pytest
  import ray
  import ray._private.ray_constants
  from ray import train
  from ray.air._internal.util import StartTraceback
  from ray.tests.conftest import pytest_runtest_makereport
  from ray.train import DataConfig
  from ray.train._internal.backend_executor import BackendExecutor, InactiveWorkerGroupError, TrainBackendError, TrainingWorkerError
  from ray.train._internal.storage import StorageContext
  from ray.train._internal.worker_group import WorkerGroup, WorkerMetadata
  from ray.train.backend import Backend, BackendConfig
  from ray.train.constants import ENABLE_SHARE_CUDA_VISIBLE_DEVICES_ENV, ENABLE_SHARE_NEURON_CORES_ACCELERATOR_ENV, TRAIN_ENABLE_WORKER_SPREAD_ENV
  from ray.train.tensorflow import TensorflowConfig
  from ray.train.torch import TorchConfig
  from ray.util.placement_group import get_current_placement_group
  from ray.util.scheduling_strategies import PlacementGroupSchedulingStrategy
label_desc: Use image node_id 2 for calling the TestBackend obj's underlying member method code with example usage: obj.on_shutdown(worker_group, backend_config) without return types