1. Luxembourg Institute of Science and Technology 1 , L-4362 Esch-sur-Alzette, Luxembourg
2. John A. Paulson School of Engineering and Applied Sciences, Harvard University 2 , Cambridge, Massachusetts 02138, USA and , Santa Clara, California 95051, USA
3. Nvidia Corporation 2 , Cambridge, Massachusetts 02138, USA and , Santa Clara, California 95051, USA
4. Engineering Laboratory, University of Cambridge 3 , Cambridge CB2 1PZ, United Kingdom