text: Title: Features to aid widespread adoption: allow any model filename, and allow specifying quantize_config at load
  Body: Firstly, thanks for all the great new commits over the last few days. AutoGPTQ is looking better and better every day!
  
  I have been doing some testing on AutoGPTQ and am really pleased to see it is now rivalling GPTQ-for-LLaMa.  As I've said before, I feel AutoGPTQ is the future of GPTQ because it is so much easier to understand and integrate.
  
  In order to help it gain widespread usage, I feel it needs some additional features:
  
  1. In `.save_quantized` and `.from_quantized`, the ability to save/load any filename.  Currently a name of `gptq_model-Xbit.bin` is mandatory. I feel that it should be possible to save with any filename, and then to load from any filename.
  2. Allow passing a BaseQuantizeConfig to `from_quantized`. This will allow loading pre-existing models that do not have a `quantize_config.json`.
  
  With these changes made, it should be easy for any user to write AutoGPTQ code that can load any of the already existing GPTQ models in existence.  For example I have many released to the community at https://huggingface.co/TheBloke, eg https://huggingface.co/TheBloke/vicuna-13B-1.1-GPTQ-4bit-128g
  
  I have tested AutoGPTQ with several of these GPTQ models (which were made with GPTQ-for-LLaMa) and they work great. But only if I manually add a quantize_config.json to each, and also rename them to gptq_model-4bit.bin.
  
  Requiring people to do this for every GPTQ model on HF will IMHO slow adoption of AutoGPTQ.  I don't mind doing it for my models, but other creators may not bother. And it'd save everyone time if it wasn't needed at all.
  
  I am happy to look into implementing these features myself as a PR, but it will likely take me a little while to figure out the code and test it properly. So I thought I would raise them as an Issue first.  Let me know what you think.
  
  Thanks again for all your great work!