Fix `main`: stable diffusion pipelines cannot be loaded (#655)

* Replace deprecation warning f-string with class name.

When `__repr__` is invoked in the instance serialization of
`config_dict` fails, because it contains `kwargs` of type `<class
inspect._empty>`.

* Revert "Replace deprecation warning f-string with class name."

This reverts commit 1c4eb8cb104374bd84e43865fc3865862473799c.

* Do not attempt to register `"kwargs"` as an attribute.

Otherwise serialization could fail.
This may happen for other attributes, so we should create a better
solution.

src/diffusers/configuration_utils.py

@@ -58,6 +58,10 @@ class ConfigMixin:
         kwargs["_class_name"] = self.__class__.__name__
         kwargs["_diffusers_version"] = __version__
 
+        # Special case for `kwargs` used in deprecation warning added to schedulers
+        # TODO: remove this when we remove the deprecation warning, and the `kwargs` argument,
+        # or solve in a more general way.
+        kwargs.pop("kwargs", None)
         for key, value in kwargs.items():
             try:
                 setattr(self, key, value)