[WIP] Add Flax FNet
# What does this PR do?

This PR adds FNet model in Flax.
Thanks for that 🤗 I could test it on TPU whenever the PR is ready, I think only the Auto Configuration stuff is missing 😅<|||||>@stefan-it It will take me some time to finish this. I will let you know when it is working!<|||||>I will be continuing this PR after #13752 is merged, just in case any changes are needed afterwards.<|||||>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.<|||||>Unstale<|||||>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.