[WIP] Add performer in flax
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR is a first draft to add the Performer: https://github.com/google-research/google-research/tree/master/performer in Trax to the library.

As stated here: https://github.com/google-research/google-research/tree/master/performer/fast_self_attention#performers-fast-self-attention-module all one has to do is to replace the `attention_fn` in `flax.linen.nn.SelfAttention` with the output of `make_fast_softmax_attention` which is done in this PR.

The function seems to integrate seamlessly with our current `FlaxBertModel` architecture, but it is questionable if the model can easily be fine-tuned on existing `BertModel` weights and if our current `FlaxBertModel` architecture corresponds to the architecture used by the Performer team to train & evaluate their models as described in the Paper.

One can verify that a forward pass of the Performer model works by running:

```python
pytest tests/test_modeling_performer.py
```

Tagging @TevenLeScao @mfuntowicz @thomwolf  for information.
@mfuntowicz - it seems like the weights can be loaded and a forward pass does throw any errors -> does that seem to work for you after taking a first look? 

## 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?
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      to the 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. Feel free to tag
members/contributors which may be interested in your PR.

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 albert, bert, XLM: @LysandreJik
 GPT2: @LysandreJik, @patrickvonplaten
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 Translation: @sshleifer
 Summarization: @sshleifer
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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 Text Generation: @patrickvonplaten, @TevenLeScao
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 T5: @patrickvonplaten
 Rag: @patrickvonplaten, @lhoestq
 EncoderDecoder: @patrickvonplaten
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 examples/seq2seq: @sshleifer
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
 documentation: @sgugger
 FSTM: @stas00
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- TF code: https://github.com/google-research/google-research/tree/master/performer/fast_attention/tensorflow


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