Prevent potential bug in DatasetFolder.make_dataset (#3733)

torchvision/datasets/folder.py

@@ -200,6 +200,13 @@ class DatasetFolder(VisionDataset):
         extensions: Optional[Tuple[str, ...]] = None,
         is_valid_file: Optional[Callable[[str], bool]] = None,
     ) -> List[Tuple[str, int]]:
+        if class_to_idx is None:
+            # prevent potential bug since make_dataset() would use the class_to_idx logic of the
+            # find_classes() function, instead of using that of the find_classes() method, which
+            # is potentially overridden and thus could have a different logic.
+            raise ValueError(
+                "The class_to_idx parameter cannot be None."
+            )
         return make_dataset(directory, class_to_idx, extensions=extensions, is_valid_file=is_valid_file)
 
     def find_classes(self, dir: str) -> Tuple[List[str], Dict[str, int]]: