Inference support for GPTQ (llama + falcon tested) + Quantization script

[Narsil]

Let's start discussing implementation.

• Need to expose the quantization scripts (either included here or add doc on how to use https://github.com/qwopqwop200/GPTQ-for-LLaMa)
• Make sure GPTQ works for multiple models (priority to Falcon).

Currently it means that every place we use get_{tensor|sharded} to check for quantization.

My idea is to reintegrate as much as possible into utils/layer.py by expanding load_multi to be a bit more generic. This might require some thinking, but ultimately the qweight,qzeros,scales,g_idx should be in a single place, and independant of bias presence.

What does this PR do?

Fixes # (issue)

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