Fabric: Enable "auto" for devices and accelerator as cli arguments

src/lightning/fabric/cli.py

@@ -34,7 +34,7 @@
 _CLICK_AVAILABLE = RequirementCache("click")
 _LIGHTNING_SDK_AVAILABLE = RequirementCache("lightning_sdk")
 
-_SUPPORTED_ACCELERATORS = ("cpu", "gpu", "cuda", "mps", "tpu")
+_SUPPORTED_ACCELERATORS = ("cpu", "gpu", "cuda", "mps", "tpu", "auto")
 
 
 def _get_supported_strategies() -> list[str]:
@@ -187,6 +187,19 @@ 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"
+
+    if devices == "auto":
+        if accelerator == "cuda" and torch.cuda.device_count() > 0 or accelerator == "mps" or accelerator == "cpu":
+            devices = "1"
+        else:
+            raise ValueError(f"Cannot default to '1' device for accelerator='{accelerator}'")
     if accelerator == "gpu":
         parsed_devices = _parse_gpu_ids(devices, include_cuda=True, include_mps=True)
     elif accelerator == "cuda":

tests/tests_fabric/test_cli.py

@@ -46,7 +46,7 @@ def test_run_env_vars_defaults(monkeypatch, fake_script):
     assert "LT_PRECISION" not in os.environ
 
 
-@pytest.mark.parametrize("accelerator", ["cpu", "gpu", "cuda", pytest.param("mps", marks=RunIf(mps=True))])
+@pytest.mark.parametrize("accelerator", ["cpu", "gpu", "cuda", "auto", pytest.param("mps", marks=RunIf(mps=True))])
 @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_accelerator(_, accelerator, monkeypatch, fake_script):