

TITLE
Add REALM

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# What does this PR do?

This PR adds REALM.

- Original paper: https://arxiv.org/abs/2002.08909
- Code and checkpoints: https://github.com/google-research/language/tree/master/language/realm

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

Closes https://github.com/huggingface/transformers/issues/3497