ORT optimizer refactorization (#294)
* Refactorization of ORTOptimizer
* Refactorization of ORTModel
* Adapt examples according to refactorization
* Adapt tests
* Fix style
* Remove quantizer modification
* Fix style
* Apply modifications from #270 for quantizer and optimizer to have same behavior
* Add test for optimization of Seq2Seq models
* Fix style
* Add ort config saving when optimizing a model
* Add ort config saving when quantizing a model
* Add tests
* Fix style
* Adapt optimization examples
* Fix readme
* Remove unused parameter
* Adapt quantization examples
* Fix quantized model and ort config saving
* Add documentation
* Add model configuration saving to simplify loading of optimized model
* Fix style
* Fix description
* Fix quantization tests
* Remove opset argument which is onnx config default opset when exporting with ORTModels
