Caffe2 graph to json for visualization in flow
Caffe2 graph to json for visualization in flow

Summary:
- Writing a Caffe2 computation graph to json for visualization in Flow
- Example use in the Text models workflow: it replaces the existing draw function which produces PNG file
- Visualization: https://our.intern.facebook.com/intern/fblearner/c2graphvis/13215753/
- The visualization uses FBLearnerDAG. Plan to add many visualization-related features.

Reviewed By: Mortimerp9

Differential Revision: D4415299

fbshipit-source-id: 2d641d60177566ed2837fb3750394420690f28de

Yangqing/caffe2,sf-wind/caffe2,xzturn/caffe2,pietern/caffe2,sf-wind/caffe2,caffe2/caffe2,sf-wind/caffe2,pietern/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,pietern/caffe2,bwasti/caffe2,davinwang/caffe2,davinwang/caffe2,Yangqing/caffe2,bwasti/caffe2,davinwang/caffe2,bwasti/caffe2,sf-wind/caffe2,xzturn/caffe2,davinwang/caffe2,davinwang/caffe2,bwasti/caffe2,Yangqing/caffe2,bwasti/caffe2,pietern/caffe2,sf-wind/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2