Summary:
Added a script to convert pkl model to pb.
- Supported Faster R-CNN detection model.
- Supported converting to CPU and GPU model.
- Fused AffineChannel to Conv.
- Verified model after conversion.

Reviewed By: rbgirshick

Differential Revision: D6783633

fbshipit-source-id: 19706d3074a4a784a2161a695d7c534e014ebb3f