llama : support RWKV v6 models

[MollySophia]

This should fix #846.

Added:

ggml:

• Added unary operation Exp
• Added rwkv_wkv operation with CPU impl
• Added rwkv_token_shift operation with CPU impl to handle multiple sequences in parallel(may not be necessary after #8526 is done)

llama.cpp:

• rwkv_world tokenizer support (by @LaylBongers)
• convert_hf_to_gguf.py support for converting RWKV v6 HF models
• RWKV v6 graph building

TODO:

• Do modifications after #8526 is ready accordingly
• Add CUDA or Metal implementation for rwkv_wkv operation

• [x] I have read the contributing guidelines
• Self-reported review complexity:
  • [x] Medium