Handle edge case for find_usable_cuda_devices(0) (#18722)

Author: awaelchli

src/lightning/fabric/CHANGELOG.md

@@ -221,6 +221,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed redundant input-type casting in FSDP precision ([#18630](https://github.com/Lightning-AI/lightning/pull/18630))
 
 
+- Fixed an issue with `find_usable_cuda_devices(0)` incorrectly returning a list of devices ([#18722](https://github.com/Lightning-AI/lightning/pull/18722))
+
+
 ## [2.0.9] - 2023-09-14
 
 ### Fixed

src/lightning/fabric/accelerators/cuda.py

@@ -90,6 +90,8 @@ def find_usable_cuda_devices(num_devices: int = -1) -> List[int]:
         both processes determine that the device is unoccupied, leading into one of them crashing later on.
 
     """
+    if num_devices == 0:
+        return []
     visible_devices = _get_all_visible_cuda_devices()
     if not visible_devices:
         raise ValueError(

tests/tests_fabric/accelerators/test_cuda.py

@@ -157,3 +157,6 @@ def test_find_usable_cuda_devices_error_handling():
         "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) == []