https://blog.csdn.net/dlchang_chang/article/details/79101960
https://blog.csdn.net/shengyan5515/article/details/83147085
focal loss:
剖析 Focal Loss 损失函数: 消除类别不平衡+挖掘难分样本
https://blog.csdn.net/Gentleman_Qin/article/details/87343004
何恺明大神的「Focal Loss」,如何更好地理解?
https://blog.csdn.net/lanyuxuan100/article/details/79055027