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
      )
  
imports: import os
  from copy import deepcopy
  from datetime import timedelta
  from unittest import mock
  from unittest.mock import MagicMock, Mock
  import pytest
  import torch
  from lightning.fabric.plugins import DoublePrecision, HalfPrecision, Precision
  from lightning.fabric.plugins.environments import LightningEnvironment
  from lightning.fabric.strategies import DDPStrategy
  from lightning.fabric.strategies.ddp import _DDPBackwardSyncControl
  from lightning.fabric.utilities.imports import _TORCH_GREATER_EQUAL_2_0
  from torch.nn.parallel import DistributedDataParallel
  from tests_fabric.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