Add MVP model (#17787)
* Add MVP model
* Update README
* Remove useless module
* Update docs
* Fix bugs in tokenizer
* Remove useless test
* Remove useless module
* Update vocab
* Remove specifying
* Remove specifying
* Add #Copied ... statement
* Update paper link
* Remove useless TFMvp
* Add #Copied ... statement
* Fix style in test mvp model
* Fix some typos
* Fix properties of unset special tokens in non verbose mode
* Update paper link
* Update MVP doc
* Update MVP doc
* Fix README
* Fix typos in docs
* Update docs
