Acknowledgment

This work was supported by the National Key Research and Development Program of China (Grant No. 2017YFA0402600), the Natural Science Foundation of Shandong (Grant No. ZR2015FL006), the CAS International Partnership Program (Grant No. 114A11KYSB20160008), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB23000000), the National Natural Science Foundation of China (Grant Nos. 61472043, 11743002, 11873067, 11690024, and 11725313), the Joint Research Fund in Astronomy (Grant No. U1531242) under Cooperative Agreement between the NSFC and CAS and National Natural Science Foundation of China (Grant No. 11673005), and the Chinese Academy of Sciences Pioneer Hundred Talents Program. The authors also thank Chavonne Bowen and Alan Ho for labeling FAST pulsar candidates, the PALFA, GBNCC team and the Arecibo Remote Command Center (ARCC) students, Cherry Ng, Meng Yu, et al. for labeling and sharing their data. The authors thank the referee's constructive comments and suggestions.

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