Summary:
Fixed "cannot find cmake/Cuda.cmake" error when executing "make ops" command (the correct path is cmake/legacy/Cuda.cmake")
Closes https://github.com/facebookresearch/Detectron/pull/399

Reviewed By: rbgirshick

Differential Revision: D7790070

Pulled By: ir413

fbshipit-source-id: 6fc49b226be227dc8e9e638a9f4c39ef2efc22aa