Add additional activation functions

[mewilhel]

• [ ] tanhshrink(x) = x - tanh(x) (concavoconvex)
• [ ] relu6(x) = min(max(0, x), 6) (convexoconcave)
• [ ] mish(x) = x * tanh(softplus(x)) (mixed convexity)
• [ ] lisht(x) = x * tanh(x)
• [ ] hardtanh(x) = max(-1, min(1, x)) (softmax)