Make `ModelOutput` serializable (#26493)

* Make `ModelOutput` serializable

Original PR from diffusers : https://github.com/huggingface/diffusers/pull/5234

* Black

src/transformers/utils/generic.py

@@ -416,6 +416,13 @@ class ModelOutput(OrderedDict):
         # Don't call self.__setattr__ to avoid recursion errors
         super().__setattr__(key, value)
 
+    def __reduce__(self):
+        if not is_dataclass(self):
+            return super().__reduce__()
+        callable, _args, *remaining = super().__reduce__()
+        args = tuple(getattr(self, field.name) for field in fields(self))
+        return callable, args, *remaining
+
     def to_tuple(self) -> Tuple[Any]:
         """
         Convert self to a tuple containing all the attributes/keys that are not `None`.