Applies the Scaled Exponential Linear Unit.
\[\begin{split}\text{selu}(x) = \begin{cases}
\lambda x & \text{if } x > 0 \\
\lambda \alpha (\exp(x) - 1) & \text{if } x \leq 0
\end{cases}\end{split}\]
where \(\lambda = 1.0507\) and \(\alpha = 1.67326\).
See also elu().
