Add CausalLM functionality to ModernBERT

[orionw]

This PR adds CausalLM functionality to ModernBERT. This means:

• Tokenizer can work in either setting
• CausalLM functions as either an MLM-based CausalLM or a normal CausalLM decoder-only.

This is for an upcoming release that trained both options for a comparison of encoders and decoders.

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,
  Pull Request section?
• 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, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

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.