Fix pytest `world_size` environment bug (#4590)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ultralytics/engine/trainer.py

@@ -168,9 +168,11 @@ class BaseTrainer:
 
     def train(self):
         """Allow device='', device=None on Multi-GPU systems to default to device=0."""
-        if isinstance(self.args.device, int) or self.args.device:  # i.e. device=0 or device=[0,1,2,3]
-            world_size = torch.cuda.device_count()
-        elif torch.cuda.is_available():  # i.e. device=None or device=''
+        if isinstance(self.args.device, str) and len(self.args.device):  # i.e. device='0' or device='0,1,2,3'
+            world_size = len(self.args.device.split(','))
+        elif isinstance(self.args.device, tuple):  # multi devices from cli is tuple type
+            world_size = len(self.args.device)
+        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'
             world_size = 0