ultralytics 8.1.40 search in Python sets {} for speed (#9450)

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/models/yolo/detect/train.py

@@ -44,7 +44,7 @@ class DetectionTrainer(BaseTrainer):
 
     def get_dataloader(self, dataset_path, batch_size=16, rank=0, mode="train"):
         """Construct and return dataloader."""
-        assert mode in ["train", "val"]
+        assert mode in {"train", "val"}, f"Mode must be 'train' or 'val', not {mode}."
         with torch_distributed_zero_first(rank):  # init dataset *.cache only once if DDP
             dataset = self.build_dataset(dataset_path, mode, batch_size)
         shuffle = mode == "train"