GenerationConfig: warn if pad token is negative (#30187)

* warn if pad token is negative

* Update src/transformers/generation/configuration_utils.py
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

* Update src/transformers/generation/configuration_utils.py
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>

* Update src/transformers/generation/configuration_utils.py
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>

---------
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>

src/transformers/generation/configuration_utils.py

@@ -472,6 +472,11 @@ class GenerationConfig(PushToHubMixin):
             raise ValueError(f"`early_stopping` must be a boolean or 'never', but is {self.early_stopping}.")
         if self.max_new_tokens is not None and self.max_new_tokens <= 0:
             raise ValueError(f"`max_new_tokens` must be greater than 0, but is {self.max_new_tokens}.")
+        if self.pad_token_id is not None and self.pad_token_id < 0:
+            warnings.warn(
+                f"`pad_token_id` should be positive but got {self.pad_token_id}. This will cause errors when batch generating, if there is padding. "
+                "Please set `pas_token_id` explicitly by `model.generation_config.pad_token_id=PAD_TOKEN_ID` to avoid errors in generation, and ensure your `input_ids` input does not have negative values."
+            )
 
         # Validation of attribute relations:
         fix_location = ""