use auto detect accelerator and add auto to the unit tests

Author: fnhirwa

src/lightning/fabric/cli.py

@@ -28,6 +28,7 @@
 from lightning.fabric.utilities.device_parser import _parse_gpu_ids
 from lightning.fabric.utilities.distributed import _suggested_max_num_threads
 from lightning.fabric.utilities.load import _load_distributed_checkpoint
+from lightning.pytorch.trainer.connectors.accelerator_connector import _AcceleratorConnector
 
 _log = logging.getLogger(__name__)
 
@@ -188,15 +189,9 @@ def _set_env_variables(args: Namespace) -> None:
 def _get_num_processes(accelerator: str, devices: str) -> int:
     """Parse the `devices` argument to determine how many processes need to be launched on the current machine."""
     if accelerator == "auto" or accelerator is None:
-        if torch.cuda.is_available() and torch.cuda.device_count() > 0:
-            accelerator = "cuda"
-        elif torch.backends.mps.is_available() and torch.backends.mps.is_built():
-            accelerator = "mps"
-        else:
-            accelerator = "cpu"
-
+        accelerator = _AcceleratorConnector._choose_auto_accelerator()
     if devices == "auto":
-        if accelerator == "cuda" and torch.cuda.device_count() > 0 or accelerator == "mps" or accelerator == "cpu":
+        if accelerator == "cuda" or accelerator == "mps" or accelerator == "cpu":
             devices = "1"
         else:
             raise ValueError(f"Cannot default to '1' device for accelerator='{accelerator}'")

tests/tests_fabric/test_cli.py

@@ -85,7 +85,7 @@ def test_run_env_vars_unsupported_strategy(strategy, fake_script):
     assert f"Invalid value for '--strategy': '{strategy}'" in ioerr.getvalue()
 
 
-@pytest.mark.parametrize("devices", ["1", "2", "0,", "1,0", "-1"])
+@pytest.mark.parametrize("devices", ["1", "2", "0,", "1,0", "-1", "auto"])
 @mock.patch.dict(os.environ, os.environ.copy(), clear=True)
 @mock.patch("lightning.fabric.accelerators.cuda.num_cuda_devices", return_value=2)
 def test_run_env_vars_devices_cuda(_, devices, monkeypatch, fake_script):
@@ -97,7 +97,7 @@ def test_run_env_vars_devices_cuda(_, devices, monkeypatch, fake_script):
 
 
 @RunIf(mps=True)
-@pytest.mark.parametrize("accelerator", ["mps", "gpu"])
+@pytest.mark.parametrize("accelerator", ["mps", "gpu", "auto"])
 @mock.patch.dict(os.environ, os.environ.copy(), clear=True)
 def test_run_env_vars_devices_mps(accelerator, monkeypatch, fake_script):
     monkeypatch.setitem(sys.modules, "torch.distributed.run", Mock())