TF Longformer (#5764)

* improve names and tests longformer

* more and better tests for longformer

* add first tf test

* finalize tf basic op functions

* fix merge

* tf shape test passes

* narrow down discrepancies

* make longformer local attn tf work

* correct tf longformer

* add first global attn function

* add more global longformer func

* advance tf longformer

* finish global attn

* upload big model

* finish all tests

* correct false any statement

* fix common tests

* make all tests pass except keras save load

* fix some tests

* fix torch test import

* finish tests

* fix test

* fix torch tf tests

* add docs

* finish docs

* Update src/transformers/modeling_longformer.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Update src/transformers/modeling_tf_longformer.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* apply Lysandres suggestions

* reverse to assert statement because function will fail otherwise

* applying sylvains recommendations

* Update src/transformers/modeling_longformer.py

Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

* Update src/transformers/modeling_tf_longformer.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

docs/source/model_doc/longformer.rst

@@ -102,3 +102,25 @@ LongformerForQuestionAnswering
 
 .. autoclass:: transformers.LongformerForQuestionAnswering
     :members:
+
+
+TFLongformerModel
+~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.TFLongformerModel
+    :members:
+
+
+TFLongformerForMaskedLM
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.TFLongformerForMaskedLM
+    :members:
+
+
+TFLongformerForQuestionAnswering
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.TFLongformerForQuestionAnswering
+    :members:
+