College of Computer Science and Technology, Huaqiao University, Jimei Avenue, Xiamen, 361021, China. wanghuazhen@hqu.edu.cn.|Automation Department, Xiamen University, Siming South Road, Xiamen, 361005, China. yang@xmu.edu.cn.|Computer Learning Research Centre, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK. zhiyuan@cs.rhul.ac.uk.