Forward path computes as ordinary threshold, and Backward path computes as a derivative from Sigmoid function.

So, I need to call only backward path of nn.functions.sigmoid in my backward path. How can I do It? I think that it’s will be more faster than self implemented derivative of Sigmoid,

To do so, you want to create your own Function where you reimplement the sigmoid backward.
It should be fairly easy as it is: grad_output * (1 - output) * output where output is the output of the forward pass and grad_output is the grad given as parameter for the backward.