Add FlaxBigBird QuestionAnswering script
# What does this PR do?

This PR will add flax-bigbird QA script on `natural-questions` dataset.

## Who can review?

Anyone in the community is free to review the PR once the tests have passed.

@patrickvonplaten 
@patrickvonplaten, this PR is ready for review & merge (tested all the code after porting here).

Failing test is unrelated to this PR.<|||||>Awesome merging for the sprint - we'll fix bugs on the go as it's under `research_projects` :-)