Added warning message when --single-cls is used incorrectly by [#13028](https://github.com/ultralytics/yolov5/issues/13028)

train.py

@@ -176,7 +176,14 @@ def train(hyp, opt, device, callbacks):
         data_dict = data_dict or check_dataset(data)  # check if None
     train_path, val_path = data_dict["train"], data_dict["val"]
     nc = 1 if single_cls else int(data_dict["nc"])  # number of classes
-    names = {0: "item"} if single_cls and len(data_dict["names"]) != 1 else data_dict["names"]  # class names
+    if single_cls and len(data_dict["names"]) != 1:
+        LOGGER.warning(
+            "WARNING ⚠️ Please check the dataset to ensure that when --single-cls is enabled, "
+            "the number of classes in the dataset is 1."
+        )
+        names = {0: "item"}  # combine all classes into a single class
+    else:
+        names = data_dict["names"]  # class names
     is_coco = isinstance(val_path, str) and val_path.endswith("coco/val2017.txt")  # COCO dataset
 
     # Model