Add Basic RWKV Block to GPT-NeoX

[Quentin Anthony]

We want to add RWKV to gpt-neox:

• Add basic RWKV block, without kernels, from https://github.com/BlinkDL/RWKV-LM to https://github.com/EleutherAI/gpt-neox/tree/main/megatron/model
• Add rwkv kernels
• Add config options for rwkv
• Add assertions to gpt-neox so that parallelism schemes and other architectures are disabled when rwkv is enabled in config