

TITLE
[WIP] Add Fusion-in-Decoder

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# What does this PR do?

This PR adds the Fusion-in-Decoder model to the repository.

Paper: https://arxiv.org/abs/2007.01282
Code: https://github.com/facebookresearch/FiD

## Who can review?
Anyone in the community is free to review the PR once the tests have passed.
@patil-suraj, @patrickvonplaten, @qqaatw