Add stable diffusion onnx runtime pipeline (#786)
* Add ONNX Runtime stable diffusion pipeline
* Fix config saving and loading
* Fix style
* Fix normalized config
* Add test
* Fix style
* Add test
* Add documentation
* Fix documentation
* Add tests
* Add diffusion model subfolder name
* Clean documentation
* Fix docstrings
* Modify image documentation
* update following reviews
* fix style
* fix imports
* add missing import
* add dummy objects for import when dependency not available
* set ORT_SUPPORTED_TASKS to empty dictionnary if ort not available
* fix test model loading
* Fix decoder model saving method
