[T5] Add open / closed book answering models
# 🌟 New model addition

## Model description
 Check here: https://github.com/google-research/google-research/tree/master/t5_closed_book_qa
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## Open source status

* [ ] the model implementation is available: (give details)
* [ ] the model weights are available: (give details)
* [ ] who are the authors: (mention them, if possible by @gh-username)

Done: https://huggingface.co/models?search=ssm