Better error message for bitsandbytes import

[SunMarc]

What does this PR do ?

This PR adds more context to the ImportError that is raised when is_bitsandbytes_available() is False. In this PR, we introduced a GPU check. Hence, is_bitsandbytes_available() can be False even though bitsandbytes is installed.
Solves issue #26165
cc @NielsRogge as this happened to you too

[ArthurZucker]

Thanks, we can be a bit more specific on which part it is no? You are not using a GPU / torch.cuda not available as it's the only compatible GPU no?

[ArthurZucker]

Thanks!