migrate add operator to the new interface (#21152)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21152
Migrate existing add benchmark to use the new op front-end
Reviewed By: zheng-xq
Differential Revision: D15325524
fbshipit-source-id: 34e969e1bd289913d881c476711bce9f8ac18a29
