build_gradient_node is not supperted for Sigmoid and Tanh.

[Walleclipse]

Thanks for the great work.

When I use the Sigmoid activation function. It raises and NotImplementedError: " Function bound_forward of BoundSigmoid(name="/18") is not supported yet"

I think it is because there is no build_gradient_node function for Sigmoid and Tanh.

Do you have the plan to implement bound computing for activations like Sigmoid, and Tanh?