code: def test_single_cpu():
      """Tests if device is set correctly for single CPU strategy."""
      trainer = Trainer(accelerator="cpu")
      assert isinstance(trainer.strategy, SingleDeviceStrategy)
      assert trainer.strategy.root_device == torch.device("cpu")
  
imports: import pickle
  from unittest.mock import MagicMock, Mock
  import pytest
  import torch
  from lightning.pytorch import Trainer
  from lightning.pytorch.core.optimizer import LightningOptimizer
  from lightning.pytorch.demos.boring_classes import BoringModel, RandomDataset
  from lightning.pytorch.strategies import SingleDeviceStrategy
  from torch.utils.data import DataLoader
  from tests_pytorch.helpers.dataloaders import CustomNotImplementedErrorDataloader
  from tests_pytorch.helpers.runif import RunIf
label_desc: Use image node_id 1 for calling a global function with example usage: test_single_cpu() without return types