LED (#9278)

* create model

* add integration

* save current state

* make integration tests pass

* add one more test

* add explanation to tests

* remove from bart

* add padding

* remove unnecessary test

* make all tests pass

* re-add cookie cutter tests

* finish PyTorch

* fix attention test

* Update tests/test_modeling_common.py

* revert change

* remove unused file

* add string to doc

* save intermediate

* make tf integration tests pass

* finish tf

* fix doc

* fix docs again

* add led to doctree

* add to auto tokenizer

* added tips for led

* make style

* apply jplus statements

* correct tf longformer

* apply lysandres suggestions

* apply sylvains suggestions

* Apply suggestions from code review

tests/test_modeling_tf_longformer.py

@@ -339,6 +339,8 @@ class TFLongformerModelTest(TFModelTesterMixin, unittest.TestCase):
 
     @slow
     def test_saved_model_with_attentions_output(self):
+        # longformer has special attentions which are not
+        # compatible in graph mode
         pass