Add OFA
Author: gchhablani
Pull request: yes

# What does this PR do?
This PR adds OFA to transformers.

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Fixes #15813 

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Comments:
Hello, glad to find you start to add `OFA` to `transformers` :tada:. I'm a member of [OFA-sys](https://github.com/OFA-Sys/OFA). If you encounter any difficulties or need any materials, please feel free to contact me. We are willing to do anything that helps.

will OFA be added to transformers models soon?

@chenxwh I will try to get the basics ready in a week where you may be able to start with the experimentation. Sorry for the delay.

Not sure if this is was a coordinated collaborative effort, but the OFA-Sys team seems to have made a preliminary release of a transformers implementation on a [separate OFA branch](https://github.com/OFA-Sys/OFA/tree/feature/add_transformers/transformers/src/transformers/models/ofa). 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.