Fix select_device() for Multi-GPU

Possible fix for #6431

utils/torch_utils.py

@@ -61,7 +61,7 @@ def select_device(device='', batch_size=0, newline=True):
     if cpu:
         os.environ['CUDA_VISIBLE_DEVICES'] = '-1'  # force torch.cuda.is_available() = False
     elif device:  # non-cpu device requested
-        nd = torch.cuda.device_count()  # number of CUDA devices
+        nd = len(os.getenv('CUDA_VISIBLE_DEVICES', '').replace(',',''))  # number of CUDA devices
         assert torch.cuda.is_available(), 'CUDA is not available, use `--device cpu` or do not pass a --device'
         assert nd > int(max(device.split(','))), f'Invalid `--device {device}` request, valid devices are 0 - {nd - 1}'
         os.environ['CUDA_VISIBLE_DEVICES'] = device  # set environment variable (must be after asserts)