Add softsign activation for v2 API.

[lcy-seso]

softsign activation:

• forward computation:
  $\text{softsign}(z) = \frac{z}{1 + |z|}$
• backward computation:
  $\frac{\partial{\text{softsign(z)}}}{\partial{z}} = \frac{1}{(1+|z|)^2}$