Add sub operator
Summary: Add sub operator for caffe2
Test Plan:
```
buck test //caffe2/torch/fb/model_transform/c2_convert:c2_pt_converter_test
```
Reviewed By: houseroad
Differential Revision: D24685090
fbshipit-source-id: 60d745065d01b634ebd3087e533d8b9ddab77a1f
