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>

tests/test_modeling_common.py

@@ -1043,7 +1043,6 @@ class ModelTesterMixin:
                 with tempfile.TemporaryDirectory() as temp_dir_name:
                     model.base_model.save_pretrained(temp_dir_name)
                     model, loading_info = model_class.from_pretrained(temp_dir_name, output_loading_info=True)
-
                     with self.subTest(msg=f"Missing keys for {model.__class__.__name__}"):
                         self.assertGreater(len(loading_info["missing_keys"]), 0)