[WIP] FNet
This PR adds FNet in PyTorch.

- Paper: https://arxiv.org/pdf/2105.03824v2.pdf
- Code and Checkpoints: https://github.com/google-research/google-research/tree/master/f_net
- Authors: @jtainslie @ilyaeck @santiaontanon-google

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/master/CONTRIBUTING.md) are likely to be ignored.<|||||>I am still working on this PR in a branch and will create another PR when it's somewhat ready.