Tianjin Key Laboratory of Cognitive Computing and Application, Tianjin University, Tianjin, 300072, China. hufeiyc@gmail.com.|Department of Computer Science and Engineering, University of California, San Diego, 92093 La Jolla, CA, USA. yul280@ucsd.edu.|Tianjin Key Laboratory of Cognitive Computing and Application, Tianjin University, Tianjin, 300072, China. jtang@ces.sc.edu.