fix: Fixed a wrong decorator by modifying it to `@classmethod` (#7653)

* Fixed wrong decorator by modifying it to @classmethod.

* Updated the method and it's argument.

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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

src/diffusers/configuration_utils.py

@@ -450,8 +450,8 @@ class ConfigMixin:
         return outputs
 
     @staticmethod
-    def _get_init_keys(cls):
-        return set(dict(inspect.signature(cls.__init__).parameters).keys())
+    def _get_init_keys(input_class):
+        return set(dict(inspect.signature(input_class.__init__).parameters).keys())
 
     @classmethod
     def extract_init_dict(cls, config_dict, **kwargs):