add transformers gptq weights

[flozi00]

same as done here huggingface/text-generation-inference#963

[tgaddair]

Thanks for the PR @flozi00! LGTM

[flozi00]

File "/opt/conda/lib/python3.9/site-packages/lorax_server/utils/weights.py", line 237, in _get_gptq_params
    bits = self.gptq_bits

AttributeError: 'Weights' object has no attribute 'gptq_bits'

still having this error, will add another PR probably but need to set up dev env for this project

[flozi00]

#55