code: def test_find_usable_cuda_devices_error_handling():
      """Test error handling for edge cases when using `find_usable_cuda_devices`."""
      # Asking for GPUs if no GPUs visible
      with mock.patch(
          "lightning.fabric.accelerators.cuda.num_cuda_devices",
          return_value=0,
      ), pytest.raises(
          ValueError,
          match="You requested to find 2 devices but there are no visible CUDA",
      ):
          find_usable_cuda_devices(2)
  
      # Asking for more GPUs than are visible
      with mock.patch(
          "lightning.fabric.accelerators.cuda.num_cuda_devices",
          return_value=1,
      ), pytest.raises(
          ValueError, match="this machine only has 1 GPUs"
      ):
          find_usable_cuda_devices(2)
  
      # All GPUs are unusable
      tensor_mock = Mock(
          side_effect=RuntimeError
      )  # simulate device placement fails
      with mock.patch(
          "lightning.fabric.accelerators.cuda.num_cuda_devices",
          return_value=2,
      ), mock.patch(
          "lightning.fabric.accelerators.cuda.torch.tensor", tensor_mock
      ), pytest.raises(
          RuntimeError,
          match=escape(
              "The devices [0, 1] are occupied by other processes"
          ),
      ):
          find_usable_cuda_devices(2)
  
      # Request for as many GPUs as there are, no error should be raised
      with mock.patch(
          "lightning.fabric.accelerators.cuda.num_cuda_devices",
          return_value=5,
      ), mock.patch("lightning.fabric.accelerators.cuda.torch.tensor"):
          assert find_usable_cuda_devices(-1) == [0, 1, 2, 3, 4]
  
      # Edge case
      assert find_usable_cuda_devices(0) == []
  
file_path: pytorch-lightning/tests/tests_fabric/accelerators/test_cuda.py
imports: import importlib
  import logging
  import os
  from re import escape
  from unittest import mock
  from unittest.mock import Mock
  import lightning.fabric
  import pytest
  import torch
  from lightning.fabric.accelerators.cuda import CUDAAccelerator, _check_cuda_matmul_precision, find_usable_cuda_devices, is_cuda_available, num_cuda_devices
  from tests_fabric.helpers.runif import RunIf
label_desc: Use image node_id 9 for calling a global function with example usage: test_find_usable_cuda_devices_error_handling() without return types