fix device defaulting for macos

Author: fnhirwa

src/lightning/fabric/cli.py

@@ -187,8 +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":
-        devices = "1"  # default to 1 device if 'auto' is specified
+        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":
@@ -197,13 +208,6 @@ def _get_num_processes(accelerator: str, devices: str) -> int:
         parsed_devices = MPSAccelerator.parse_devices(devices)
     elif accelerator == "tpu":
         raise ValueError("Launching processes for TPU through the CLI is not supported.")
-    elif accelerator == "auto" or accelerator is None:
-        if torch.cuda.is_available() and torch.cuda.device_count() > 0:
-            parsed_devices = CUDAAccelerator.parse_devices(devices)
-        elif torch.backends.mps.is_available() and torch.backends.mps.is_built():
-            parsed_devices = MPSAccelerator.parse_devices(devices)
-        else:
-            return CPUAccelerator.parse_devices(devices)
     else:
         return CPUAccelerator.parse_devices(devices)
     return len(parsed_devices) if parsed_devices is not None else 0