[Flax] Fix BigBird

[patrickvonplaten]

What does this PR do?

PyTorch BigBird was changed in a recent PR: ba1b3db but the Flax version wasn't changed accordingly.

Thanks for spotting it @sgugger

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,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? 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, and
  here are tips on formatting docstrings.
• 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 who may be interested in your PR.

[sgugger]

Thanks for fixing! On the equivalence tests, would it make sense to have one in the Flax common tests and one in the PyTorch common tests? This would have caught the problem on the original PR.

[patrickvonplaten]

I see -> yeah we can do this!