[`bnb`] 8bit models should not be converted to `DDP`
# What does this PR do?

Fix issues that users can encounter on multi-GPU such as: https://github.com/huggingface/peft/issues/269#issuecomment-1498776567 

In fact 8bit models should not be converted to DDP

cc @sgugger @pacman100 
_The documentation is not available anymore as the PR was closed or merged._