[Flax/JAX] Run jitted tests at every commit
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Thanks to @sgugger's PR to run only tests that are affected by the code change, we can re-enable jitted Flax/JAX tests at every commit in my opinion. A jitted Flax/JAX test takes between 20 seconds and 5 minutes per model (only BigBird takes 5 minutes, the second longest test takes 1min), so a total of around 20 minutes (only when files affected all Flax models are pushed). 

If just a single Flax model is changed the tests will take a minute or so, see: https://circle-production-customer-artifacts.s3.amazonaws.com/picard/forks/5bdabdd888af1f000130874a/226201271/6113ef4861c2ff26950fd762-0-build/artifacts/~/transformers/reports/tests_flax_durations.txt?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Date=20210811T154534Z&X-Amz-SignedHeaders=host&X-Amz-Expires=60&X-Amz-Credential=AKIAJR3Q6CR467H7Z55A%2F20210811%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Signature=1eae0d3553c5ebf6daac2dfa0db9af4dde96e7fe1630bab6d16e442eb8832cb7 

@sgugger I think it's fine to run the jitted Flax/JAX tests now everytime thanks to your PR :-)

## Before submitting
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## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
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- 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
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Comments:
@sgugger - I'll wait with merging this PR hten until "efficient testing" is rolled out for self-push github action

Efficient tests are rolled out on `self-push` -> merging the PR