Make DatasetFolder.find_classes public (#3628)

Co-authored-by: Nicolas Hug <nicolashug@fb.com>

torchvision/datasets/folder.py

@@ -182,7 +182,7 @@ class DatasetFolder(VisionDataset):
     ) -> None:
         super(DatasetFolder, self).__init__(root, transform=transform,
                                             target_transform=target_transform)
-        classes, class_to_idx = self._find_classes(self.root)
+        classes, class_to_idx = self.find_classes(self.root)
         samples = self.make_dataset(self.root, class_to_idx, extensions, is_valid_file)
 
         self.loader = loader
@@ -202,8 +202,12 @@ class DatasetFolder(VisionDataset):
     ) -> List[Tuple[str, int]]:
         return make_dataset(directory, class_to_idx, extensions=extensions, is_valid_file=is_valid_file)
 
-    @staticmethod
-    def _find_classes(dir: str) -> Tuple[List[str], Dict[str, int]]:
+    def find_classes(self, dir: str) -> Tuple[List[str], Dict[str, int]]:
+        """Same as :func:`find_classes`.
+
+        This method can be overridden to only consider
+        a subset of classes, or to adapt to a different dataset directory structure.
+        """
         return find_classes(dir)
 
     def __getitem__(self, index: int) -> Tuple[Any, Any]: