1. School of Instrument Science and Engineering, Harbin Institute of Technology (HIT), Harbin, China
2. Department of Information Engineering and Computer Science, University of Trento, Trento, TN, Italy
3. Department of Computer Science and Engineering, Hong Kong University of Science and Technology, Hong Kong
4. SAP AI Research, Berlin, Germany
5. Perception Group, INRIA, Le Chesnay-Rocquencourt, France