Add `accelerate` support for BART-like models
# What does this PR do?

This PR adds `accelerate` support for BART-like models, so that these models can be loaded in 8bit using `load_in_8bit=True`.
Follows the same logic as https://github.com/huggingface/transformers/pull/19912 regarding shared embeddings

Do not merge before https://github.com/huggingface/accelerate/pull/792 gets merged! 

cc @sgugger 


_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks a lot! Merging since https://github.com/huggingface/accelerate/pull/792 has been merged 🟢 