Deprecate/Remove GPTQ.py

[jerryzh168]

Currently it's used by ExecuTorch I think, we should deprecate the APIs in GPTQ.py and move to new APIs

[jerryzh168]

cc @andrewor14 @HDCharles

[andrewor14]

I think @telgamal-1 is still recently using this for llama quantization. We probably need to keep this path around until they migrate to the subclass flow