Add BigBirdPegasus (#10991)

* init bigbird pegasus

* add debugging nb ; update config

* init conversion

* update conversion script

* complete conversion script

* init forward()

* complete forward()

* add tokenizer

* add some slow tests

* commit current

* fix copies

* add docs

* add conversion script for bigbird-roberta-summarization

* remove TODO

* small fixups

* correct tokenizer

* add bigbird core for now

* fix config

* fix more

* revert pegasus-tokenizer back

* make style

* everything working for pubmed; yayygit status

* complete tests finally

* remove bigbird pegasus tok

* correct tokenizer

* correct tests

* add tokenizer files

* finish make style

* fix test

* update

* make style

* fix tok utils base file

* make fix-copies

* clean a bit

* small update

* fix some suggestions

* add to readme

* fix a bit, clean tests

* fix more tests

* Update src/transformers/__init__.py

* Update src/transformers/__init__.py

* make fix-copies

* complete attn switching, auto-padding left

* make style

* fix auto-padding test

* make style

* fix batched attention tests

* put tolerance at 1e-1 for stand-alone decoder test

* fix docs

* fix tests

* correct slow tokenizer conversion

* Apply suggestions from code review
Co-authored-by: Suraj Patil <surajp815@gmail.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* complete remaining suggestions

* fix test
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
Co-authored-by: Suraj Patil <surajp815@gmail.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

utils/check_repo.py

@@ -35,6 +35,9 @@ PATH_TO_DOC = "docs/source"
 # Being in this list is an exception and should **not** be the rule.
 IGNORE_NON_TESTED = [
     # models to ignore for not tested
+    "BigBirdPegasusEncoder",  # Building part of bigger (tested) model.
+    "BigBirdPegasusDecoder",  # Building part of bigger (tested) model.
+    "BigBirdPegasusDecoderWrapper",  # Building part of bigger (tested) model.
     "M2M100Encoder",  # Building part of bigger (tested) model.
     "M2M100Decoder",  # Building part of bigger (tested) model.
     "Speech2TextEncoder",  # Building part of bigger (tested) model.