fix a bug in `from_config` (#7192)

* fix

* fix

* update comment

---------
Co-authored-by: yiyixuxu <yixu310@gmail,com>

src/diffusers/configuration_utils.py

@@ -259,6 +259,10 @@ class ConfigMixin:
         model = cls(**init_dict)
 
         # make sure to also save config parameters that might be used for compatible classes
+        # update _class_name
+        if "_class_name" in hidden_dict:
+            hidden_dict["_class_name"] = cls.__name__
+
         model.register_to_config(**hidden_dict)
 
         # add hidden kwargs of compatible classes to unused_kwargs