Add apply_yarn_scaling as optional key to yarn

[juliendenize]

What does this PR do?

This PR adds apply_yarn_scaling as an optional key for yarn repo.

This was requested as part of a vLLM PR vllm-project/vllm#37104 that seeks to silence some rope issues when converting Mistral config to HF one.

cc @hmellor

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[hmellor]

LGTM, let's get a second review from a Transformers team member though

[juliendenize]

LGTM, let's get a second review from a Transformers team member though

yes, maybe @ArthurZucker @Cyrilvallez are the most suited for this ?

[hmellor]

Sorry for the back and forth @juliendenize, I have just found that there is a better solution we can implement on the vLLM side.

[juliendenize]

Yes thanks