Added Windows cmd to count GPU devices (ultralytics#7891)

* Added Windows cmd to count GPU devices

* Cleanup

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/torch_utils.py

@@ -40,10 +40,10 @@ def torch_distributed_zero_first(local_rank: int):
 
 
 def device_count():
-    # Returns number of CUDA devices available. Safe version of torch.cuda.device_count(). Only works on Linux.
-    assert platform.system() == 'Linux', 'device_count() function only works on Linux'
+    # Returns number of CUDA devices available. Safe version of torch.cuda.device_count(). Supports Linux and Windows
+    assert platform.system() in ('Linux', 'Windows'), 'device_count() only supported on Linux or Windows'
     try:
-        cmd = 'nvidia-smi -L | wc -l'
+        cmd = 'nvidia-smi -L | wc -l' if platform.system() == 'Linux' else 'nvidia-smi -L | find /c /v ""'  # Windows
         return int(subprocess.run(cmd, shell=True, capture_output=True, check=True).stdout.decode().split()[-1])
     except Exception:
         return 0