Enhance mixed-bit exporting for GPTQ format

[wenhuach21]

GPTQModel introduces the dynamic key to flexibly configure mixed-bit settings

[JartX]

Hi @wenhuach21 check It please :) vllm-project/vllm#23994

[wenhuach21]

Hi @wenhuach21 check It please :) vllm-project/vllm#23994

@JartX Thank you for your efforts in supporting AutoRound

[JartX]

@wenhuach21 It wasn't because hehe, it was in case it could help you to identify, thanks to you for autoround! :)