1. JSPS Research Fellow, Ichibancho 8, Chiyoda‐ku, Tokyo 102‐8472, Japan
2. Neural Computation Unit, Okinawa Institute of Science and Technology, 1919‐1 Tancha, Onna‐son, Kunigami, Okinawa 904‐0412, Japan
3. Graduate School of Information Science and Technology, The University of Tokyo, Hongo 7‐3‐1, Bunkyo‐ku, Tokyo 113‐8656, Japan
4. Research Center for Advanced Science and Technology, The University of Tokyo, Komaba 4‐6‐1, Meguro‐ku, Tokyo 153‐8904, Japan
5. PRESTO, JST, 4‐1‐8 Honcho Kawaguchi, Saitama 332‐0012, Japan