1. Center for Big Data Research, Peking University, 12465 Beijing, China, 100871 (e-mail: jiny@stu.pku.edu.cn)
2. Computer Vision, Tencent AI Lab, 546374 Shenzhen, Guangdong, China, (e-mail: cswhjiang@gmail.com)
3. QCIS, University of Technology, Sydney, Ultimo, New South Wales, Australia, 2007 (e-mail: Yi.Yang@uts.edu.au)
4. Wangxuan Institute of Computer Technology, Peking University, Beijing, China, 100080 (e-mail: muyadong@gmail.com)