Remove usage of torch._six (#7243)

Author: NicolasHug

torchvision/datasets/utils.py

@@ -451,7 +451,7 @@ def verify_str_arg(
     valid_values: Optional[Iterable[T]] = None,
     custom_msg: Optional[str] = None,
 ) -> T:
-    if not isinstance(value, torch._six.string_classes):
+    if not isinstance(value, str):
         if arg is None:
             msg = "Expected type str, but got type {type}."
         else:

torchvision/datasets/vision.py

@@ -36,7 +36,7 @@ def __init__(
         target_transform: Optional[Callable] = None,
     ) -> None:
         _log_api_usage_once(self)
-        if isinstance(root, torch._six.string_classes):
+        if isinstance(root, str):
             root = os.path.expanduser(root)
         self.root = root