bitsandbytes: simplify 8bit dequantization

[matthewdouglas]

What does this PR do?

Simplifies the dequantization for bitsandbytes int8 weights. There is a similar PR open in huggingface/peft#2245.

In the upcoming bitsandbytes release we will have an added API for for int8 dequantization. For backwards compatibility, a simplified (but functionally equivalent) dequantization operation is performed.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@SunMarc @BenjaminBossan

[HuggingFaceDocBuilderDev]

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[BenjaminBossan]

LGTM, thanks. The failing test tests/models/xlm_roberta_xl/test_modeling_xlm_roberta_xl.py::XLMRobertaXLModelTest::test_assisted_decoding_matches_greedy_search_1_same appears to be unrelated.

[SunMarc]

LGTM ! Left a question

[ArthurZucker]

A welcome change! THanks 🤗