add weight property to exllamav2 quanlinear

[flozi00]

What does this PR do?

Just recognized that merging is not successfull, so added the property as travis did the last time to exllama v1

Fixes # (issue)

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

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