Fix BigBirdModelTester
# What does this PR do?

Current `BigBirdModelTester` with `block_size=16` will cause the model to change `attention_type` from `block_sparse` to `original_full`.

https://github.com/huggingface/transformers/blob/9fef668338b15e508bac99598dd139546fece00b/src/transformers/models/big_bird/modeling_big_bird.py#L2059-L2070

In the PT/Flax equivalence test (from `ModelTesterMixin`), the `FlaxBigBird` model will still use `block_sparse`. This causes the test failed with `1e-5`.

I couldn't find any word `not possible` or `impossible` in `modeling_flax_big_bird.py`.

Currently, I change  `block_size=16` to  `block_size=8` in order to make both models run with `block_sparse` and pass the test.
However, it would be good at some point to check if we should have the same `self.set_attention_type("original_full")` for `FlaxBigBirdModel`.

Question: is `block_size=16` chosen intentionally to test `self.set_attention_type` in PT's BigBird?
_The documentation is not available anymore as the PR was closed or merged._<|||||>The expected value in `test_fast_integration` needs to be updated - if the change in this PR is OK.<|||||>Totally fine to switch to `block_size=8` if both test run the `"block_sparse"` mode this way :-) Thanks a lot for looking into this!<|||||>Update the expected value in `test_fast_integration` - to take into account of the change `block_size=8` and therefore `block_sparse`.