Missing `Optional` in typing. (#30821)

The function checks for None in its first line.

src/transformers/generation/utils.py

@@ -1255,7 +1255,7 @@ class GenerationMixin:
         return generation_config
 
     def _prepare_generation_config(
-        self, generation_config: GenerationConfig, **kwargs: Dict
+        self, generation_config: Optional[GenerationConfig], **kwargs: Dict
     ) -> Tuple[GenerationConfig, Dict]:
         """
         Prepares the base generation config, then applies any generation configuration options from kwargs.