code: def test_ddp_process_group_backend(
      process_group_backend, device_str, expected_process_group_backend
  ):
      """Test settings for process group backend."""
  
      class MockDDPStrategy(DDPStrategy):
          def __init__(self, root_device, process_group_backend):
              self._root_device = root_device
              super().__init__(
                  process_group_backend=process_group_backend
              )
  
          @property
          def root_device(self):
              return self._root_device
  
      strategy = MockDDPStrategy(
          process_group_backend=process_group_backend,
          root_device=torch.device(device_str),
      )
      assert (
          strategy._get_process_group_backend()
          == expected_process_group_backend
      )
  
file_path: pytorch-lightning/tests/tests_pytorch/strategies/test_ddp.py
imports: import os
  from datetime import timedelta
  from unittest import mock
  import pytest
  import torch
  from lightning.fabric.plugins.environments import LightningEnvironment
  from lightning.fabric.utilities.imports import _TORCH_GREATER_EQUAL_2_0
  from lightning.pytorch import LightningModule, Trainer
  from lightning.pytorch.demos.boring_classes import BoringModel
  from lightning.pytorch.plugins import DoublePrecision, HalfPrecision, Precision
  from lightning.pytorch.strategies import DDPStrategy
  from lightning.pytorch.trainer.states import TrainerFn
  from torch.nn.parallel import DistributedDataParallel
  from tests_pytorch.helpers.runif import RunIf
label_desc: Use image node_id 1 for calling a global function with example usage: test_ddp_process_group_backend(process_group_backend, device_str, expected_process_group_backend) and returns: self