For rotated proposals, replace cv::rotatedRectangleIntersection with a correct versio...

For rotated proposals, replace cv::rotatedRectangleIntersection with a correct version that doesn't have underflow problem (#15113)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15113

cv::rotatedRectangleIntersection has a known float underflow bug that would cause failure in ```CV_Assert(intersection.size() <= 8)```

For rotated proposals, replace cv::rotatedRectangleIntersection with a correct version that doesn't have underflow problem.

Otherwise, when ```USE_CPP_GENERATE_PROPOSALS = true```, the training would fail.

Reviewed By: viswanathgs

Differential Revision: D13429770

fbshipit-source-id: 5e95d059f3c668f14059a0a83e8e53d8554cdb99