Function selu 

pub fn selu<const D: usize, B>(tensor: Tensor<B, D>) -> Tensor<B, D>
where
    B: Backend,

Applies the Scaled Exponential Linear Unit function element-wise as described in the paper Self-Normalizing Neural Networks.

selu(x) = gamma * x if x > 0, gamma * alpha * (exp(x) - 1) if x <= 0