1. Tsinghua University & Baidu Research, Beijing, China
2. Northwestern Polytechnical University & Baidu Research, Beijing, China
3. Zhejiang University & Baidu Research, Beijing, China
4. Baidu Research, Beijing, China
5. Institute of Information Engineering, Chinese Academy of Sciences, Beijing, China
6. Northwestern Polytechnical University, Xi'an, China
7. Baidu Research, Sunnyvale, CA, USA