 ONNX conversion script.
This PR adds conversion script to export our models to ONNX IR. 

Plan is to support both PyTorch and TensorFlow: 

- [x] PyTorch
- [ ] TensorFlow

TensorFlow currently blocked because of an issue in the conversion script provided by ONNX (seems fixed on master : https://github.com/onnx/tensorflow-onnx/issues/876)