Adding OPTForSeqClassification class (#18123)
* Adding OPTForSeqClassification class
* Fix import issues
* Add documentation for optforseqclassification
* Remove checkout
* fix failing tests
* fix typo
* Fix code formatting
* Incorporating the PR feedbacks
* Incorporate PR Feedbacks
* Fix failing test and add new test for multi label setup
* Fix formatting issue
* Fix failing tests
* Fix formatting issues
* Fix failing tests
* Fix failing tests
* Fix failing tests
* Fix failing tests
* PR feedback
