A Hierarchical Reinforcement Learning Based Optimization Framework for Large-scale Dynamic Pickup and Delivery Problems - Ma, Yi; Hao, Xiaotian; Hao, Jianye; Lu, Jiawen; Liu, Xing; Xialiang, Tong; Yuan, Mingxuan; Li, Zhigang; Tang, Jie; Meng, Zhaopeng (Advances in Neural Information Processing Systems, 2021)