[`Nllb-Moe`] Fix nllb moe accelerate issue
# What does this PR do?

Fixes: https://github.com/huggingface/transformers/issues/23385 

Before this PR, it seemed that `_no_split_modules` was not properly set. Due to the skip connections in `NllbMoeEncoderLayer` and `NllbMoeDecoderLayer` one needs to add these modules inside `_no_split_modules` instead of `NllbMoeAttention`.

All accelerate tests pass

cc @ArthurZucker @sgugger 

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