Add DeepSeek V2 Model into Transformers

[VladOS95-cyber]

What does this PR do?

This PR add DeepSeekV2 into the Transformers library. There is a new thing in transformers called modular, which adds new models by creating a modeling_modelname.py file. Since DeepSeekV2 could reuse some Llama arch parts, it serves as an ideal use case for this modular approach.

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. Link: Deepseek v2 #35317
• 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 @Rocketknight1