only enable FloatToHalf test for GPU
only enable FloatToHalf test for GPU

Reviewed By: bddppq

Differential Revision: D6945312

fbshipit-source-id: 9550a9607c0daec6783ce63d3c9f082ff27b0303

Yangqing/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,caffe2/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2