Add "Leveraging Pretrained Checkpoints for Generation" Seq2Seq models. (#6594)

* add conversion script

* improve conversion script

* make style

* add tryout files

* fix

* update

* add causal bert

* better names

* add tokenizer file as well

* finish causal_bert

* fix small bugs

* improve generate

* change naming

* renaming

* renaming

* renaming

* remove leftover files

* clean files

* add fix tokenizer

* finalize

* correct slow test

* update docs

* small fixes

* fix link

* adapt check repo

* apply sams and sylvains recommendations

* fix import

* implement Lysandres recommendations

* fix logger warn

docs/source/index.rst

@@ -134,7 +134,10 @@ conversion utilities for the following models:
 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
+27. `Bert For Sequence Generation <https://tfhub.dev/s?module-type=text-generation&subtype=module,placeholder>`_ (from Google) released with the paper
+    `Leveraging Pre-trained Checkpoints for Sequence Generation Tasks
+    <https://arxiv.org/abs/1907.12461>`_ by Sascha Rothe, Shashi Narayan, Aliaksei Severyn.
+28. `Other community models <https://huggingface.co/models>`_, contributed by the `community
     <https://huggingface.co/users>`_.
 
 .. toctree::
@@ -221,6 +224,7 @@ conversion utilities for the following models:
     model_doc/mbart
     model_doc/funnel
     model_doc/lxmert
+    model_doc/bertgeneration
     internal/modeling_utils
     internal/tokenization_utils
     internal/pipelines_utils