BartForCausalLM analogs to ProphetNetForCausalLM (#9128)

* initiliaze bart4causalLM

* create BartDecoderWrapper, setters/getters

* delete spaces

* forward and additional methods

* update cache function, loss function, remove ngram* params in data class.

* add bartcausallm, bartdecoder testing

* correct bart for causal lm

* remove at

* add mbart as well

* up

* fix typo

* up

* correct

* add pegasusforcausallm

* add blenderbotforcausallm

* add blenderbotsmallforcausallm

* add marianforcausallm

* add test for MarianForCausalLM

* add Pegasus test

* add BlenderbotSmall test

* add blenderbot test

* fix a fail

* fix an import fail

* a fix

* fix

* Update modeling_pegasus.py

* fix models

* fix inputs_embeds setting getter

* adapt tests

* correct repo utils check

* finish test improvement

* fix tf models as well

* make style

* make fix-copies

* fix copies

* run all tests

* last changes

* fix all tests

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

docs/source/model_doc/bart.rst

@@ -130,6 +130,12 @@ BartForQuestionAnswering
 .. autoclass:: transformers.BartForQuestionAnswering
     :members: forward
 
+BartForCausalLM
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.BartForCausalLM
+    :members: forward
+
 
 
 TFBartModel