Evaluate ONNX training proposal

[chinhuang007]

We would like to evaluate the feasibility of the ONNX training proposal, https://github.com/onnx/onnx/issues/2038, and share our results with the Training WG and converter SIG.

The goal of this issue/task is to identify the proposed ONNX training spec can be practically generated and used in Tensorflow and other frameworks for training by

1. Investigate and provide prototype code for conversion from ONNX training IR to Tensorflow (and others if possible) trainable model format

2. Investigate and provide prototype code for conversion from Tensorflow (and others if possible) trainable model format to ONNX training IR