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

docs/source/index.rst

@@ -240,6 +240,8 @@ TensorFlow and/or Flax.
 +-----------------------------+----------------+----------------+-----------------+--------------------+--------------+
 |     Funnel Transformer      |       ✅       |       ✅       |       ✅        |         ✅         |      ❌      |
 +-----------------------------+----------------+----------------+-----------------+--------------------+--------------+
+|             LED             |       ✅       |       ✅       |       ✅        |         ✅         |      ❌      |
++-----------------------------+----------------+----------------+-----------------+--------------------+--------------+
 |           LXMERT            |       ✅       |       ✅       |       ✅        |         ✅         |      ❌      |
 +-----------------------------+----------------+----------------+-----------------+--------------------+--------------+
 |          LayoutLM           |       ✅       |       ✅       |       ✅        |         ❌         |      ❌      |
@@ -371,6 +373,7 @@ TensorFlow and/or Flax.
     model_doc/fsmt
     model_doc/funnel
     model_doc/layoutlm
+    model_doc/led
     model_doc/longformer
     model_doc/lxmert
     model_doc/marian