Support InfiniAI Megrez 3b

[dixyes]

This pr is to add InfiniAI Megrez support into llama.cpp

The model now(@58f1df16523cb2a9acb225aa808146e052f2b5b2) seems have a wrong eos_token set in its tokenizer_config.json.
( <|turn_end|> at template and <|turn_end> in json) Not sure if this is on purpose. Also metioned here

So the converted model will not stop generating in chat mode. Modify it to <|turn_end|> in tokenizer_config.json, then the generated gguf will work.

[ngxson]

I'm not sure about the tokenizer_pre == "megrez" part (if other collaborators know, please feel free to review this PR).

The template part looks good to me.

[arch-btw]

Thanks for doing this, I was trying it myself but didn't finish it. Just so you know they fixed the eos 30 minutes ago.