CLI DDP fixes (#135)

ultralytics/yolo/utils/torch_utils.py

@@ -29,10 +29,11 @@ WORLD_SIZE = int(os.getenv('WORLD_SIZE', 1))
 @contextmanager
 def torch_distributed_zero_first(local_rank: int):
     # Decorator to make all processes in distributed training wait for each local_master to do something
-    if local_rank not in {-1, 0}:
+    initialized = torch.distributed.is_initialized()  # prevent 'Default process group has not been initialized' errors
+    if initialized and local_rank not in {-1, 0}:
         dist.barrier(device_ids=[local_rank])
     yield
-    if local_rank == 0:
+    if initialized and local_rank == 0:
         dist.barrier(device_ids=[0])