Implement BigBird in TensorFlow
Author: E-Aho
Pull request: yes

# What does this PR do?

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Fixes #19430 by implementing BigBird in TF


## 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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- [ ] Did you write any new necessary tests? 
(WRITING TESTS IN PROGRESS)


## 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.

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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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PR implements BigBird based on implementations of:
* [Original BigBird implementation](https://github.com/google-research/bigbird/blob/master/bigbird/core/attention.py)
* [PyTorch BigBird implementation in PyTorch](https://github.com/huggingface/transformers/blob/main/src/transformers/models/big_bird/modeling_big_bird.py)
* [TF version of Bert](https://github.com/huggingface/transformers/blob/main/src/transformers/models/bert/modeling_tf_bert.py)

Raising this as a draft PR while I work on tests and ironing out issues I run into while testing, but thought it might be useful to let others have visibility of this while working on it!

Comments:
_The documentation is not available anymore as the PR was closed or merged._