Add q-cache 6 and 8 support for Exllamav2

[randoentity]

Checklist:

• I have read the Contributing guidelines.

[GodEmperor785]

@oobabooga could this be merged to main? It would be useful for models that can get unstable with Q4 cache quantization (like Qwen or Mistral Nemo as reported by some people), also the current 8bit cache seems to be old and author of exllamav2 says that Q8 is better (even Q4 can be better while taking less space).