Methods for ranking
https://github.com/MountLee/btl_mle_l_inf
Kernel smmothing method: https://github.com/shamindras/bttv-aistats2020
Temporal regularization method: https://github.com/shamindras/bradley-terry-convexopt
Based on Bradley-Terry model: https://github.com/MountLee/CPD_BT