Add Youtu-LLM model

[LuJunru]

What does this PR do?

This PR adds the implementation for the released Youtu-LLM model. The model has the following features:

• Type: Autoregressive Causal Language Models with Dense MLA
• Release versions: Base and Instruct

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?

@ArthurZucker @Cyrilvallez

[github-actions]

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

run-slow: auto, youtu_llm

[github-actions]

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=43165&sha=0797aa

[LuJunru]

The base repository was forked 8 years ago from the transformers repo of that time, and further synchronized with commits, resulting in branch naming issues. I will close this pull request and working with a new fork.

The new pr is here: #43166.