Add export to ONNX

[SthPhoenix]

Describe the changes you've made

Add script model2onnx.py to export keras model to ONNX

Describe if there is any unusual behaviour of your code(Write NA if there isn't)

NA

Additional context (OPTIONAL)

With proposed script it's possible to export model to ONNX which can be used in ONNXRuntime or TensorRT, both were tested.

Test plan (OPTIONAL)

Checklist

• [x] My code follows the code style of this project.
• [x] I have performed a self-review of my own code.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [x] My changes generate no new warnings.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [x] The title of my pull request is a short description of the requested changes.