Add ONNXRuntime Python interface

[zhiqwang]

🚀 The feature

Add ONNXRuntime Python interface.

Motivation, pitch

My current plan is to put the core code for ORT inference into the directory yolort/runtime. And furthermore we could name the python scripts as ort_modeling.py, you can also use others if you have a better suggestion. We plan to add more python inferencing backends in the future in this directory. And then add a CLI tool named run_model.py (just for example) to the directory tools for use cases.

cc @itsnine

Originally posted by @zhiqwang in #176 (comment)