Fix DDP when device is a list (#4600)

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

tests/test_cuda.py

@@ -28,7 +28,14 @@ def test_checks():
 @pytest.mark.skipif(not CUDA_IS_AVAILABLE, reason='CUDA is not available')
 def test_train():
     device = 0 if CUDA_DEVICE_COUNT == 1 else [0, 1]
-    YOLO(MODEL).train(data=DATA, imgsz=64, epochs=1, batch=-1, device=device)  # also test AutoBatch, requires imgsz>=64
+    YOLO(MODEL).train(data=DATA, imgsz=64, epochs=1, device=device)  # requires imgsz>=64
+
+
+@pytest.mark.skipif(not CUDA_IS_AVAILABLE, reason='CUDA is not available')
+def test_autobatch():
+    from ultralytics.utils.autobatch import check_train_batch_size
+
+    check_train_batch_size(YOLO(MODEL).model.cuda(), imgsz=128, amp=True)
 
 
 @pytest.mark.skipif(not CUDA_IS_AVAILABLE, reason='CUDA is not available')

ultralytics/engine/trainer.py

@@ -164,7 +164,7 @@ class BaseTrainer:
         """Allow device='', device=None on Multi-GPU systems to default to device=0."""
         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
+        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 torch.cuda.is_available():  # i.e. device=None or device='' or device=number
             world_size = 1  # default to device 0