Fix exllama wronfully loading

[maximelaboisson]

What does this PR do?

The changes disabling exllama on old compute had unintended consequences of not setting use_exllama to False if HAS_EXLLAMA equals False and CAN_EXLLAMA equals False. This fixes this.

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?

@OlivierDehaene @Narsil

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[Narsil]

Thanks a lot !