Clean Encoder-Decoder models with Bart/T5-like API and add generate possibility (#3383)

* change encoder decoder style to bart & t5 style

* make encoder decoder generation dummy work for bert

* make style

* clean init config in encoder decoder

* add tests for encoder decoder models

* refactor and add last tests

* refactor and add last tests

* fix attn masks for bert encoder decoder

* make style

* refactor prepare inputs for Bert

* refactor

* finish encoder decoder

* correct typo

* add docstring to config

* finish

* add tests

* better naming

* make style

* fix flake8

* clean docstring

* make style

* rename

docs/source/index.rst

@@ -89,6 +89,7 @@ The library currently contains PyTorch and Tensorflow implementations, pre-train
     :caption: Package Reference
 
     model_doc/auto
+    model_doc/encoderdecoder
     model_doc/bert
     model_doc/gpt
     model_doc/transformerxl