Flax T5
# What does this PR do?

This PR will add T5 in Flax/Jax.

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


## Who can review?

Anyone in the community is free to review the PR once the tests have passed.

@patrickvonplaten
Exceptionally merging already given the time constraint of the Flax/JAX community week announcement. 
@patil-suraj @sgugger, I would be very happy if you could nevertheless take a look after merge so that I can correct suggestions in a follow-up PR before the next transformers release.<|||||>Is a jax/flax byT5 planned?

Interested in both byT5 and jax... torn.<|||||>You can already use ByT5 in jax/flax! Check the models [here](https://huggingface.co/models?filter=jax&search=byt5)<|||||>oh, my 🤗<|||||>I'm having trouble finding the Jax model training and architecture definition.

Is this just loading a byT5 model into a regular T5 inference scaffolding?

My aim is to experiment with the training / masking code.<|||||>Here is a test for FlaxByT5 that could help a bit: https://github.com/huggingface/transformers/blob/332a2458611751e7d9c4d7a21bc454299d50e160/tests/test_modeling_flax_t5.py#L432

Also we have `run_summarization` script in Flax that can be easily tweaked for any seq2seq task. And soon (Monday) we'll have FlaxT5 pretraining as well :-) 