PegasusX add _no_split_modules
Author: andreeahedes
Pull request: yes

# What does this PR do?
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Fixes # (issue)
Add in no_split_modules method for PegasusX model to allow disk/cpu offloading and multi-GPU parallelism. 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      to it if that's the case. ->https://github.com/huggingface/accelerate/issues/1900
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Comments:
@andreeahedes Thanks for adding this! Have you run the [accelerate tests](https://huggingface.co/docs/transformers/testing#run-accelerate-tests) for this model on 1 and 2 GPUs with these changes? 

Hi @amyeroberts , I ran `test_disk_offload`, `test_cpu_offload` and ` test_model_parallelism` on a machine with 2 GPUs.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25933). All of your documentation changes will be reflected on that endpoint.