Add Cohere2 model

[alexrs-cohere]

What does this PR do?

Add Cohere2 model to transformers.

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

[ArthurZucker]

Nice! 🔥

[ArthurZucker]

let's improve this!

[guangy10]

@alexrs-cohere Thanks for adding cohere2 with test for export to ExecuTorch!