Add FAN Model 
# What does this PR do?

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Fixes #17234

Implements the FAN Models described in this [paper](https://arxiv.org/pdf/2204.12451.pdf) and available in the following [github repo](https://github.com/NVlabs/FAN), Additionally this repo has some of the weights available as described in their README file.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [X] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [X] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [X] Did you write any new necessary tests?


This is a cleanup from previous PR #20288 in order to mantain branch integrity, recommendations by @NielsRogge were implemented


## 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.

@NielsRogge, @sgugger, @patrickvonplaten 

## Additional Request

If this PR gets merged, would it be possible to migrate the model files from [my HF space](https://huggingface.co/ksmcg) to the nvidia space

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Hi @sgugger thanks for you're feedback. I'll try to implement the changes soon<|||||>Implemented suggestions by @sgugger.

Pending the change on the README.md path, since I'm uncertain if I need to change only the README.md path or the actual doc path.

Also pending rebase<|||||>Thanks for working on this! You need to change the link to the doc in the READMEs as suggested, but not the path to the file. You will also need to rebase/resolve the conflicts.

@NielsRogge could you have a review before I do a final pass?<|||||>I've applied the README.md update and rebased the branch.
<|||||>Hi @NielsRogge, @sgugger.

First of all happy new year, I hope 2023 is greater success than 2022 was for the huggingface team.
I've resolved the merge conflicts, and was hoping to know if any additional steps were required for this PR?
<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20417). All of your documentation changes will be reflected on that endpoint.<|||||>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/main/CONTRIBUTING.md) are likely to be ignored.