onnxruntime example package for MNIST handwritten digit prediction

This package is the 1st example showing the usage of onnxruntime in athena. It is based on !30886 (closed) and !31209 (closed) developped by @akraszna and me. It has full demonstration from loading a .onnx model to data loading and prediction of output. Adding @akraszna , @afarbin and @dguest.