[Docs] Add DialoGPT (#3755)

* add dialoGPT

* update README.md

* fix conflict

* update readme

* add code links to docs

* Update README.md

* Update dialo_gpt2.rst

* Update pretrained_models.rst

* Update docs/source/model_doc/dialo_gpt2.rst

Co-Authored-By: Julien Chaumond <chaumond@gmail.com>

* change filename of dialogpt

Co-authored-by: Julien Chaumond <chaumond@gmail.com>

docs/source/model_doc/flaubert.rst

@@ -20,6 +20,8 @@ of the time they outperform other pre-training approaches. Different versions of
 evaluation protocol for the downstream tasks, called FLUE (French Language Understanding Evaluation), are shared
 to the research community for further reproducible experiments in French NLP.*
 
+The original code can be found `here <https://github.com/getalp/Flaubert>`_.
+
 
 FlaubertConfig
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~