Fix handle_legacy_interface when passing strings (#5063)

Author: NicolasHug

torchvision/prototype/models/_utils.py

@@ -43,6 +43,7 @@ def inner_wrapper(*args: Any, **kwargs: Any) -> M:
                 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