Fixed area calculation for rotated boxes for coco
Fixed area calculation for rotated boxes for coco

Summary:
Before, we converted the rotated box to an axis-align box and then calculated the area.
However this will overstate the area since the axis-align box will we larger then the rotated one and therefore the area will be incorrect.

Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/2529

Reviewed By: theschnitz

Differential Revision: D26038584

Pulled By: ppwwyyxx

fbshipit-source-id: 61328f4a2550ad6251595a925396bd2d0f025266

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2