[Generation, Gemma 3] When passing a custom generation_config, overwrite default values with the model's base generation_config

src/transformers/generation/utils.py

@@ -1177,21 +1177,35 @@ def _merge_criteria_processor_list(
         default_list: Union[LogitsProcessorList, StoppingCriteriaList],
         custom_list: Union[LogitsProcessorList, StoppingCriteriaList],
     ) -> Union[LogitsProcessorList, StoppingCriteriaList]:
+        """
+        Merge user-defined processors/criteria with the ones instantiated inside `generate`. In case the same
+        processor/criteria is present on both lists, use the user-defined one.
+        """
         if len(custom_list) == 0:
             return default_list
+
+        final_list = type(default_list)()
         for default in default_list:
+            using_custom = False
             for custom in custom_list:
                 if type(custom) is type(default):
                     object_type = "stopping criteria" if isinstance(custom, StoppingCriteria) else "logits processor"
-                    raise ValueError(
-                        f"A custom {object_type} of type {type(custom)} with values {custom} has been passed to"
-                        f" `.generate()`, but it has already been created with the values {default}. {default} has been"
-                        " created by passing the corresponding arguments to generate or by the model's config default"
-                        f" values. If you just want to change the default values of {object_type} consider passing"
-                        f" them as arguments to `.generate()` instead of using a custom {object_type}."
+                    logger.warning_once(
+                        f"A custom {object_type} of type {type(custom)} has been passed to `.generate()`, but it "
+                        f"was also created in `.generate()`, given its parameterization. The custom {type(custom)} "
+                        f"will take precedence. Please check the docstring of {type(custom)} to see related "
+                        "`.generate()` flags."
                     )
-        default_list.extend(custom_list)
-        return default_list
+                    final_list.append(custom)
+                    using_custom = True
+                    break
+            if not using_custom:
+                final_list.append(default)
+
+        for custom in custom_list:
+            if custom not in final_list:
+                final_list.append(custom)
+        return final_list
 
     def compute_transition_scores(
         self,
@@ -1574,16 +1588,12 @@ def _prepare_generation_config(
         if not is_torchdynamo_compiling():
             generation_config = copy.deepcopy(generation_config)
             model_kwargs = generation_config.update(**kwargs)
-            # If `generation_config` is provided, let's fallback ALL special tokens to the default values for the model
+            # If `generation_config` is provided, let's fallback ALL default values to the model's generation config
             if not using_model_generation_config:
-                if generation_config.bos_token_id is None:
-                    generation_config.bos_token_id = self.generation_config.bos_token_id
-                if generation_config.eos_token_id is None:
-                    generation_config.eos_token_id = self.generation_config.eos_token_id
-                if generation_config.pad_token_id is None:
-                    generation_config.pad_token_id = self.generation_config.pad_token_id
-                if generation_config.decoder_start_token_id is None:
-                    generation_config.decoder_start_token_id = self.generation_config.decoder_start_token_id
+                default_generation_config = GenerationConfig()
+                for key, default_value in default_generation_config.__dict__.items():
+                    if getattr(generation_config, key) == default_value:
+                        setattr(generation_config, key, getattr(self.generation_config, key))
         else:
             model_kwargs = kwargs