1. Microsoft Research Asia, Beijing, China
2. School of Computer Science and Technology, Central South University, Changsha, China
3. School of Information Science and Technology, Beijing Foreign Studies University, Beijing, China
4. Research Institute of Artificial Intelligence, Zhejiang Lab, Hangzhou, China
5. Department of Computer Science and Engineering, Hong Kong University of Science and Technology, Hong Kong
6. Department of Computer Science, University of Illinois at Chicago, Chicago, IL, USA