[RWKV] Add RWKV6 model

[uniartisan]

What does this PR do?

This is currently a draft. I will supplement the documentation and related tests as soon as possible. RWKV6 and RWKV5 are derived from the same source, and I believe it is necessary to skip RWKV5 and only support RWKV6. After the RWKV7 pre-trained model is released, I will add support for RWKV7.

Fixes # (issue)

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[Rocketknight1]

Hi @uniartisan, this looks good, but can we rework the PR to use the new Modular style? It should make the PR much smaller and easier to write/review, since you can just import most of it from other models, including earlier RWKV models.

[uniartisan]

Hi @uniartisan, this looks good, but can we rework the PR to use the new Modular style? It should make the PR much smaller and easier to write/review, since you can just import most of it from other models, including earlier RWKV models.

I can have a try later:)
The main issue is that I'm not very familiar with the previous RWKV models, though perhaps for the future RWKV7 I could reuse code from the current RWKV6. After I complete the tests and address other issues, I can try converting it to the modular style.