参考https://blog.csdn.net/kangyi411/article/details/78969642
Sigmoid
\sigma(x) = \frac {1} {1 + e^{-x}}
求导为\sigma(x)(1 - \sigma(x))
tanh
\tanh(x) = \frac{e^x - e^{-x}}{e^x + e^{-x}}
求导为1 - \tanh^2(x)
relu
f(x) = max\{0, x\}
参考https://blog.csdn.net/kangyi411/article/details/78969642
\sigma(x) = \frac {1} {1 + e^{-x}}
求导为\sigma(x)(1 - \sigma(x))
\tanh(x) = \frac{e^x - e^{-x}}{e^x + e^{-x}}
求导为1 - \tanh^2(x)
f(x) = max\{0, x\}