Funnel transformer (#6908)

* Initial model

* Fix upsampling

* Add special cls token id and test

* Formatting

* Test and fist FunnelTokenizerFast

* Common tests

* Fix the check_repo script and document Funnel

* Doc fixes

* Add all models

* Write doc

* Fix test

* Initial model

* Fix upsampling

* Add special cls token id and test

* Formatting

* Test and fist FunnelTokenizerFast

* Common tests

* Fix the check_repo script and document Funnel

* Doc fixes

* Add all models

* Write doc

* Fix test

* Fix copyright

* Forgot some layers can be repeated

* Apply suggestions from code review

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/modeling_funnel.py

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

* Address review comments

* Update src/transformers/modeling_funnel.py

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* Address review comments

* Update src/transformers/modeling_funnel.py

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

* Slow integration test

* Make small integration test

* Formatting

* Add checkpoint and separate classification head

* Formatting

* Expand list, fix link and add in pretrained models

* Styling

* Add the model in all summaries

* Typo fixes

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

docs/source/index.rst

@@ -131,7 +131,10 @@ conversion utilities for the following models:
 25. `LXMERT <https://github.com/airsplay/lxmert>`_ (from UNC Chapel Hill) released with the paper `LXMERT: Learning
     Cross-Modality Encoder Representations from Transformers for Open-Domain Question
     Answering <https://arxiv.org/abs/1908.07490>`_ by Hao Tan and Mohit Bansal.
-26. `Other community models <https://huggingface.co/models>`_, contributed by the `community
+26. `Funnel Transformer <https://github.com/laiguokun/Funnel-Transformer>`_ (from CMU/Google Brain) released with the paper
+    `Funnel-Transformer: Filtering out Sequential Redundancy for Efficient Language Processing
+    <https://arxiv.org/abs/2006.03236>`_ by Zihang Dai, Guokun Lai, Yiming Yang, Quoc V. Le.
+27. `Other community models <https://huggingface.co/models>`_, contributed by the `community
     <https://huggingface.co/users>`_.
 
 .. toctree::
@@ -216,6 +219,7 @@ conversion utilities for the following models:
     model_doc/dpr
     model_doc/pegasus
     model_doc/mbart
+    model_doc/funnel
     model_doc/lxmert
     internal/modeling_utils
     internal/tokenization_utils