[BitsAndBytesConfig] Warning for unused kwargs & safety checkers for load_in_4bit and load_in_8bit

[benjaminye]

What does this PR do?

Fixes #29736

• Introduces a helpful warning when passing in unused keyword arguments when initializing BitsAndBytesConfig
• Checks if load_in_4bit and load_in_4bit is indeed a boolean value in order to prevent arbitrary types to be passed in
• Checks for type safety again when setters for load_in_4bit and load_in_8bit are invoked

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case. (link)
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

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.

[amyeroberts]

Additions look good to me - thanks for adding!

Let's just have a final confirmation from @younesbelkada that it's OK

[younesbelkada]

Looking great thanks !

[benjaminye]

@amyeroberts @younesbelkada
Thanks everyone! 😃