Fix: Add missing pad_token_id to StableLmConfig

[tobyliu2004]

What does this PR do?

Fixes #43572

Adds the missing pad_token_id parameter to StableLmConfig to resolve the AttributeError when creating StableLM models from config.

Changes made:

• Added pad_token_id parameter to __init__ method (defaults to None)
• Added self.pad_token_id = pad_token_id assignment
• Updated docstring to document the new parameter

Testing:

Verified that AutoModelForCausalLM.from_config(StableLmConfig()) now works without errors.

Before submitting

• This PR fixes a bug
• Code follows existing patterns (matches bos_token_id and eos_token_id)
• Documentation updated in docstring

cc @ArthurZucker @Cyrilvallez @itazap

[mario-aws]

• Do we also need a test to cover such usage example?
• Do we need to update existing configs like https://huggingface.co/stabilityai/stablelm-3b-4e1t/blob/main/config.json? likely not

[tobyliu2004]

Thanks for reviewing!
You're right, I think a test would be a good idea, I've just added an automated test in test_modeling_stablelm.py that verifies StableLmConfig has the pad_token_id attribute and that models can be instantiated from config without errors. As for the existing configs, I agree that since pad_token_id is defaulted to None that they should work without updates.

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: stablelm

[Rocketknight1]

cc @zucchini-nlp

[zucchini-nlp]

Thanks for the PR @tobyliu2004 ! I'd prefer to close it in favor of #43592 which is more complete

[tobyliu2004]

Thanks for reviewing! Makes sense to fix all models at once rather than one by one. Appreciate the feedback!