function Selu() {
	    return _Activation2.apply(this, arguments) || this;
	  }
Scaled Exponential Linear Unit. (Klambauer et al., 2017).
Reference: Self-Normalizing Neural Networks, https://arxiv.org/abs/1706.02515
Notes:
  - To be used together with the initialization "lecunNormal".
  - To be used together with the dropout variant "AlphaDropout".