docs: Replace package abbreviations with full name(bitsandbytes) in docstrings

[rapsealk]

What does this PR do?

Hello, transformers team!

While setting up a new environment for LLM fine-tuning, I encountered an error message stating: Trainer tried to instantiate bnb optimizer but bnb is not installed!. I recognized that bnb refers to the bitsandbytes package and resolved the issue by installing bitsandbytes. However, this could be confusing for others who might not be aware that bnb stands for bitsandbytes.

To improve clarity and help users identify and install the correct package, I propose updating the docstrings to replace abbreviations like bnb with the full name bitsandbytes. This change will provide clearer guidance and reduce potential confusion when encountering similar error messages or documentation references.

Thanks for the review in advance!

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?

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.

[LysandreJik]

Sure, thanks @rapsealk!