Summary:
OpenCV 2, 3, 4 have different signatures for this function. Using [-2] is compatible with them.
Pull Request resolved: https://github.com/facebookresearch/Detectron/pull/847

Reviewed By: rbgirshick

Differential Revision: D14703716

Pulled By: ir413

fbshipit-source-id: 7329c5d45a0b0d2a3125198e6ac64bc79b42ff46