Add Mixture of Tokens model
Author: szysad
Pull request: yes
Labels: name: New model

# What does this PR do?

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Fixes https://github.com/huggingface/transformers/issues/28285


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @muellerzr and @pacman100

Integrations:

- deepspeed: HF Trainer/Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization (bitsandbytes, autogpt): @SunMarc and @younesbelkada

Documentation: @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: See Models above and tag the person corresponding to the modality of the example.
- TensorFlow: @Rocketknight1

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Comments:
@ArthurZucker We consider our work done, and we are ready for the review :). Failed tests seem unrelated to this code.

I performed some additional cleanup and extended the tests to cover additional cases.
The issue with the paper and the original code is available at https://github.com/huggingface/transformers/issues/28285).
The model weights are available at https://huggingface.co/jaszczur/mixture_of_tokens. This is a previously unreleased checkpoint created in cooperation with the paper authors.
We updated the documentation with our changes.
The test failures are flaky tests unrelated to our changes.
Please review and advise whether this looks good.