[`BitsAndBytesConfig`] Warning for unused `kwargs` & safety checkers for `load_in_4bit` and `load_in_8bit`
Author: benjaminye
Pull request: yes

# What does this PR do?

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Fixes [#29736](https://github.com/huggingface/transformers/issues/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
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Comments:
@amyeroberts @younesbelkada
Thanks everyone! 😃