[FlaxBert] Fix non-broadcastable attention mask for batched forward-passes
# What does this PR do?

Fixes #8790


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to 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?

@mfuntowicz 
@avital 
@LysandreJik 

@mfuntowicz @avital
I just fixed the bug that I was hitting. There might be other places that need this fix as well.<|||||>Wuuhu - first Flax PR :-). This looks great to me @KristianHolsheimer - think you touched all necessary files as well! 

@mfuntowicz - maybe you can take a look as well<|||||>**EDIT** I re-enabled GPU memory preallocation but set the mem fraction < 1/parallelism. That seemed to fix the tests. The problem with this is that future tests might fail if a model doesn't fit in 1/8th of the GPU memory.

---

The flax tests time out. When I ran the tests locally with `pytest -n auto`, I did notice OOM issues due to preallocation of GPU memory by XLA. I addressed this in commit 6bb1f5e600cd35c712f4f980699df7735b4f59eb.

Other than that, it's hard to debug the tests when there's no output.

Would it be an option to run these tests single-threaded instead?<|||||>I had the changes in another branche I'm working on, happy to merge this one and will rebase mine 👍.

Thanks for looking at it @KristianHolsheimer 