Set pad_token_id as nullable & add support for rope_parameters

[alvarobartt]

What does this PR do?

This PR fixes how the config.json is read, since the pad_token_id can be set to null which was breaking before as it was either not provided and set to 0 for CamemBERT, XLM-Roberta or Roberta, or provided and used as-is. Additionally, this PR also adds the rope_parameters key which contains a nested dictionary with the RoPE configuration which was before at the top-level dictionary in config.json, see huggingface/transformers#39847.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the documentation guidelines.
• Did you write any new necessary tests? If applicable, did you include or update the insta snapshots?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.