Fix `handle_legacy_interface` when passing strings (#5063)

torchvision/prototype/models/_utils.py

@@ -43,6 +43,7 @@ def handle_legacy_interface(**weights: Tuple[str, Union[Optional[W], Callable[[D
                 if (
                     (weights_param not in kwargs and pretrained_param not in kwargs)
                     or isinstance(weights_arg, WeightsEnum)
+                    or (isinstance(weights_arg, str) and weights_arg != "legacy")
                     or weights_arg is None
                 ):
                     continue