Use 5e-5 For BigBird PT/Flax equivalence tests
# What does this PR do?

Use `5e-5` For BigBird PT/Flax equivalence tests to avoid flaky test failure.

Also change the name `check_outputs` to `check_pt_flax_outputs` (similar to `check_pt_tf_outputs`) and update its logic similar to `check_pt_tf_outputs`. 

_The documentation is not available anymore as the PR was closed or merged._