[Flax][WIP] Speed up pretraining
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes # (issue)


## Before submitting
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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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Library:

- benchmarks: @patrickvonplaten
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- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
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HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
The whole random masking can be done before each training loop to speed things up -> would be an interesting experiment

Ok this gives no speed-up actually -> closing