Add PLBart (#13269)
* Init PLBART
* Add missing configuration file
* Add conversion script and configurationf ile
* Fix style
* Update modeling and conversion scripts
* Fix scale embedding in config
* Add comment
* Fix conversion script
* Add classification option to conversion script
* Fix vocab size in config doc
* Add tokenizer files from MBart50
* Allow no lang code in regular tokenizer
* Add PLBart Tokenizer Converters
* Remove mask from multi tokenizer
* Remove mask from multi tokenizer
* Change from MBart-50 to MBart tokenizer
* Fix names and modify src/tgt behavior
* Fix imports for tokenizer
* Remove <mask> from multi tokenizer
* Fix style
* Change tokenizer_class to processor_class
* Add attribute map to config class
* Update modeling file to modified MBart code
* Update configuration file to MBart style configuration
* Fix tokenizer
* Separate tokenizers
* Fix error in tokenization auto
* Copy MBart tests
* Replace with MBart tokenization tests
* Fix style
* Fix language code in multi tokenizer
* Fix configuration docs
* Add entry for plbart_multi in transformers init
* Add dummy objects and fix imports
* Fix modeling tests
* Add TODO in config
* Fix copyright year
* Fix modeling docs and test
* Fix some tokenization tests and style
* Add changes from review
* Fix copies
* Fix docs
* Fix docs
* Fix style
* Fix year
* Add changes from review
* Remove extra changes
* Fix base tokenizer and doc
* Fix style
* Fix modeling and slow tokenizer tests
* Remove Multi-tokenizer Converter and Tests
* Delete QA model and Multi Tokenizer dummy objects
* Fix repo consistency and code quality issues
* Fix example documentation
* Fix style
* Remove PLBartTokenizer from type checking in init
* Fix consistency issue
* Add changes from review
* Fix style
* Remove PLBartTokenizerFast
* Remove FastTokenizer converter
* Fix AutoTokenzier mapping
* Add plbart to toctree and fix consistency issues
* Add language codes tokenizer test
* Fix styling and doc issues
* Add fixes for failing tests
* Fix copies
* Fix failing modeling test
* Change assert to assertTrue in modeling tests
