Fix missing output_attentions in PT/Flax equivalence test (#16271)
* fix - set output_attentions to True
* Update tests/test_modeling_flax_common.py
* update for has_attentions
* overwrite check_outputs in FlaxBigBirdModelTest
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>
Co-authored-by: Suraj Patil <surajp815@gmail.com>