Use higher value for hidden_size in Flax BigBird test
# What does this PR do?

#17658 changed `hidden_size` from `32` to `4` in `FlaxBigBirdModelTester`, which cause the PT/Flax difference increased by ~ 50 times. This PR changes it back (but keep other changes untouched). We can therefore use `1e-5` instead of `5e-5`.

(`hidden_size=4` with `num_attention_heads=2` is likely to introduce some edge cases in random init.)

The testing time (on GCP CPU VM)  is 66 vs. 64 seconds (with `-n 1`) and 46 vs. 44 seconds (with `-n 2`), so this change doesn't make the test slower.
_The documentation is not available anymore as the PR was closed or merged._<|||||>@patil-suraj  If you have one minute to review this one, I can merge it today 😄 🙏 but not urgent