Improve trainer DDP device handling (#15383)

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

ultralytics/engine/trainer.py

@@ -174,9 +174,11 @@ class BaseTrainer:
             world_size = len(self.args.device.split(","))
         elif isinstance(self.args.device, (tuple, list)):  # i.e. device=[0, 1, 2, 3] (multi-GPU from CLI is list)
             world_size = len(self.args.device)
+        elif self.args.device in {"cpu", "mps"}:  # i.e. device='cpu' or 'mps'
+            world_size = 0
         elif torch.cuda.is_available():  # i.e. device=None or device='' or device=number
             world_size = 1  # default to device 0
-        else:  # i.e. device='cpu' or 'mps'
+        else:  # i.e. device=None or device=''
             world_size = 0
 
         # Run subprocess if DDP training, else train normally