1. School of Information Science and Engineering, Shandong University, Qingdao, China
2. Key Laboratory of Intelligent Perception and Systems for High-Dimensional Information, School of Computer Science and Engineering, Ministry of Education, Nanjing University of Science and Technology, Nanjing, China
3. Institute of Artificial Intelligence, Beihang University, Beijing, China
4. Microsoft Corporation, Bing, Bellevue, WA, USA
5. School of Electrical and Data Engineering, University of Technology Sydney, Sydney, NSW, Australia