[Bart] Refactor - fix issues, consistency with the library, naming (#8900)

* remove make on the fly linear embedding

* start refactor

* big first refactor

* save intermediate

* save intermediat

* correct mask issue

* save tests

* refactor padding masks

* make all tests pass

* further refactor

* make pegasus test pass

* fix bool if

* fix leftover tests

* continue

* bart renaming

* delete torchscript test hack

* fix imports in tests

* correct shift

* fix docs and repo cons

* re-add fix for FSTM

* typo in test

* fix typo

* fix another typo

* continue

* hot fix 2 for tf

* small fixes

* refactor types linting

* continue

* finish refactor

* fix import in tests

* better bart names

* further refactor and add test

* delete hack

* apply sylvains and lysandres commens

* small perf improv

* further perf improv

* improv perf

* fix typo

* make style

* small perf improv

docs/source/model_doc/bart.rst

@@ -105,8 +105,6 @@ BartModel
 .. autoclass:: transformers.BartModel
     :members: forward
 
-.. autofunction:: transformers.models.bart.modeling_bart._prepare_bart_decoder_inputs
-
 
 BartForConditionalGeneration
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~