[`bnb`] Fix bnb skip modules
# What does this PR do?

Fixes https://github.com/huggingface/transformers/issues/24037

https://github.com/huggingface/transformers/pull/23479 removed by mistake the logic introduced in https://github.com/huggingface/transformers/pull/21579 to deal with modules that are not needed to be converted

The PR also adds a nice test to make sure this will never happen again


_The documentation is not available anymore as the PR was closed or merged._