[Flax] Big FlaxBert Refactor
Author: patrickvonplaten
Pull request: yes
Labels: name: Flax

# What does this PR do?

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This PR does a major refactor of FlaxBert in Transformers, notably:
- Costume LayerNorm and Embedding layers are replaced by the official ones. This should significantly reduce maintenance cost at the expense of two more general lines in the conversion script.
- A couple of bugs are fixed, *e.g.*, BERT uses the *non-approximated* GELU and not the fast one -> this fixes some minor differences when comparing PyTorchBERT vs FlaxBERT
- Weight Tying is added, which should be done for, *e.g.* `FlaxBertForMaskedLM`
- Weights can now also be converted the other way around Flax => PyTorch

Sorry for putting quite a lot of things into one PR, but there are very much intertwined here.

Also, I will have to re-upload some flax weights so that they correspond to the new weight structure (see [here](https://github.com/huggingface/transformers/pull/10977))

@avital @marcvanzee, I had an issue when saving/loading flax weights for which I opened an issue [here](https://github.com/google/flax/issues/1261). At the moment, I solve it by manually transforming every numpy array into a jax DeviceArray, see [here](https://github.com/huggingface/transformers/pull/11364/files#r617853008) - not sure if there is a better solution.

## 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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## Who can review?

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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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