Flax Big Bird (#11967)

* add flax bert

* bert -> bigbird

* original_full ported

* add debugger

* init block sparse

* fix copies ; gelu_fast -> gelu_new

* block sparse port

* fix block sparse

* block sparse working

* all ckpts working

* fix-copies

* make quality

* init tests

* temporary fix for FlaxBigBirdForMultipleChoice

* skip test_attention_outputs

* fix

* gelu_fast -> gelu_new ; fix multiple choice model

* remove nsp

* fix sequence classifier

* fix

* make quality

* make fix-copies

* finish

* Delete debugger.ipynb

* Update src/transformers/models/big_bird/modeling_flax_big_bird.py

* make style

* finish

* bye bye jit flax tests

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

docs/source/index.rst

@@ -305,7 +305,7 @@ Flax), PyTorch, and/or TensorFlow.
 +-----------------------------+----------------+----------------+-----------------+--------------------+--------------+
 |       Bert Generation       |       ✅       |       ❌       |       ✅        |         ❌         |      ❌      |
 +-----------------------------+----------------+----------------+-----------------+--------------------+--------------+
-|           BigBird           |       ✅       |       ✅       |       ✅        |         ❌         |      ❌      |
+|           BigBird           |       ✅       |       ✅       |       ✅        |         ❌         |      ✅      |
 +-----------------------------+----------------+----------------+-----------------+--------------------+--------------+
 |       BigBirdPegasus        |       ❌       |       ❌       |       ✅        |         ❌         |      ❌      |
 +-----------------------------+----------------+----------------+-----------------+--------------------+--------------+