focal loss softmax cross entropy sigmoid mse loss及其求导
FL(pt)=−αt(1−pt)γlog(pt),pt=p if y=1,else (1−p)FL(p_t) = - \alpha_t (1-p_t)^{\gamma} \log{(p_t)}, p_t=p \ if \ y=1,else \ (1-p)FL(pt)=−αt(1−pt)γlog(pt),pt=p if y=1,else (1−p)论文中α=0.25\alpha=
