ORTModel uses optimum.exporters.onnx (#490)
* ORTModel uses optimum.exporters.onnx
* Refactoring
* Temp
* Almost ready
* Started fixing modeling_seq2seq.py
* Fix
* Fix
* Fix
* Improved TasksManager.determine_framework
* Fix failing tests
* Fix subfolder failing test
* Update docstring and abstract class
* Add suggestion
* Applied modification to modeling_decoder.py
* Applied suggestions
* Applied suggestions
