[`bnb`] fix `bnb` decoders bug
# What does this PR do?

Currently on the `main` branch, there is a silent bug with `bnb` and encoder-decoder models leading to some modules not being converted in `int8` for these models and hurting users that are using the `main` branch - such as `peft`: https://github.com/huggingface/peft/issues/108 

With https://github.com/huggingface/transformers/pull/21579 being introduced, the check to know if we should keep the module as `nn.Linear` has been slightly changed and [being more robust ](https://github.com/huggingface/transformers/blob/7f1cdf18958efef6339040ba91edb32ae7377720/src/transformers/utils/bitsandbytes.py#L126).

Before #21579 the function `get_keys_to_not_convert` used to return `['decoder', 'lm_head', 'wo']` which is wrong, and leading to all decoder layers being not converted in int8 with the new check as mentioned above.

This PR fixes the bug that was in this function, and adds a new test to make sure this will never happen

Fixes: https://github.com/huggingface/peft/issues/108

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