[PyTorch Bart] Split Bart into different models (#9343)
* first try
* remove old template
* finish bart
* finish mbart
* delete unnecessary line
* init pegasus
* save intermediate
* correct pegasus
* finish pegasus
* remove cookie cutter leftover
* add marian
* finish blenderbot
* replace in file
* correctly split blenderbot
* delete "old" folder
* correct "add statement"
* adapt config for tf comp
* correct configs for tf
* remove ipdb
* fix more stuff
* fix mbart
* push pegasus fix
* fix mbart
* more fixes
* fix research projects code
* finish docs for bart, mbart, and marian
* delete unnecessary file
* correct attn typo
* correct configs
* remove pegasus for seq class
* correct peg docs
* correct peg docs
* finish configs
* further improve docs
* add copied from statements to mbart
* fix copied from in mbart
* add copy statements to marian
* add copied from to marian
* add pegasus copied from
* finish pegasus
* finish copied from
* Apply suggestions from code review
* make style
* backward comp blenderbot
* apply lysandres and sylvains suggestions
* apply suggestions
* push last fixes
* fix docs
* fix tok tests
* fix imports code style
* fix doc
