Fix cache_position initialisation for generation with `use_cache=Fa…

…lse` (#30485)

* Fix cache_position init for generation

* Update src/transformers/generation/utils.py

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* Fix cache position update

---------

Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

src/transformers/generation/utils.py

@@ -667,7 +667,11 @@ def _update_model_kwargs_for_generation(
                     dim=-1,
                 )
 
-        if "cache_position" in model_kwargs and model_kwargs["cache_position"] is not None:
+        if (
+            model_kwargs.get("use_cache", True)
+            and "cache_position" in model_kwargs
+            and model_kwargs["cache_position"] is not None
+        ):
             model_kwargs["cache_position"] = model_kwargs["cache_position"][-1:] + num_new_tokens
 
         return model_kwargs
@@ -1293,6 +1297,10 @@ def _prepare_generation_config(
 
     def _get_initial_cache_position(self, input_ids, model_kwargs):
         """Calculates `cache_position` for the pre-fill stage based on `input_ids` and optionally past length"""
+        if not model_kwargs.get("use_cache", True):
+            model_kwargs["cache_position"] = None
+            return model_kwargs
+
         past_length = 0
         if "past_key_values" in model_kwargs:
             if isinstance(model_kwargs["past_key_values"], Cache):