[bnb] Small improvements on utils
# What does this PR do?

Fixes a small typo in `bitsandbytes.py`, should address https://github.com/huggingface/blog/pull/463#discussion_r946067141 
I will have to test it first and mark it as ready for review!
_The documentation is not available anymore as the PR was closed or merged._<|||||>Can confirm the tests pass!<|||||>so will there always be just one module not to convert?

won't it be safer to have modules instead and work with the list?<|||||>I have proposed a small refactoring that includes:

- checking the list of modules to not convert instead of a single value. 
- changing an error message as it confused some user. Check: https://github.com/TimDettmers/bitsandbytes/issues/10 

The bnb slow tests are passing with this fix!<|||||>From https://github.com/huggingface/transformers/issues/18660 I also just added a commit to support having a custom list of the keys to ignore <|||||>Thanks a lot @stas00 !
There is no rush at all for this PR, we can definitely wait for @sgugger before moving forward with it <|||||>Can confirm the bnb slow tests are passing with the proposed fixes! Would love to have a final round of review 💪  
cc @sgugger @stas00 <|||||>Can confirm the slow tests pass after rebasing on `main`, will merge once it's green! 🟢 