Supporting seq2seq models for `bitsandbytes` integration
# What does this PR do?

The previous checks to check which key to not convert in int8 was not sufficient. It appears that T5 models were not able to be converted correctly. This PR addresses this issue by adding an extra check consisting of checking whether the model has tied_weights inside the `get_key_to_not_convert` function. 

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