Add a feature to merge the PTH format with the original LLM directly.

[LiyanJin]

I need a LLM in the GGUF format. I have noticed that llama.cpp can convert files from the HF format to the GGUF format, and it can also convert files from the PTH format to the GGUF format. Therefore, I suggest that you add a feature to Xtuner that supports directly merging the PTH format with the original LLM.