[Flax] Adapt Flax models to new structure
Author: patrickvonplaten
Pull request: yes
Labels: name: Flax

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As discussed in https://github.com/huggingface/transformers/pull/9172, Flax model should get a design that is most similar to PyTorch and thus should use `def setup(...)` instead of `nn.compact(...)`. This PR refactors the model architecture of Bert & Roberta accordingly.

The next step is now to add a general conversion method flax<>pytorch which might require some more follow-up changes to the naming of the weights.


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Comments:
Will wait until https://github.com/huggingface/transformers/pull/10775 is merged, then rebase and then merge.

@patrickvonplaten 

I like the new structure but it seems this PR broke the flax example: https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_mlm_flax.py

- This line (https://github.com/huggingface/transformers/blob/896d7be97401a85dc0ffc5460afd707e8e092781/examples/language-modeling/run_mlm_flax.py#L577) will raise the error
```
TypeError: __init__() got an unexpected keyword argument 'dropout_rate'
```

- In addition, this line https://github.com/huggingface/transformers/blob/896d7be97401a85dc0ffc5460afd707e8e092781/src/transformers/models/bert/modeling_flax_bert.py#L254 uses an undefined variable `self.dropout_rate`.

I think we should make more test cases and make sure the examples are runnable. 


I am very interested in the jax/flax integration. Could you also take a look at my PR? https://github.com/huggingface/transformers/pull/10796
If you are collaborative and welcome contributions from me, I can contribute more and improve the flax examples.