Add RWKV models

[guangyusong]

Changes:

Added support for RWKV model family.

Related links:

Paper: https://arxiv.org/abs/2305.13048
Github: https://github.com/BlinkDL/RWKV-LM

Screenshots:

Model list:

Sample generation:

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @guangyusong to sign the Salesforce Inc. Contributor License Agreement.

[bdqnghi]

thank you. The RWKV model family looks very nice. However, they are not really code learning models, do you have the checkpoints related to coding tasks?

[guangyusong]

The RWKV model family should have a similar data split as GPT-J. We anticipate releasing a model that's more adept at coding tasks in the near future.