VL_NNSIGMOID - CNN sigmoid nonlinear unit.
Y = VL_NNSIGMOID(X) computes the sigmoid of the data X. X can have an arbitrary size. The sigmoid is defined as follows:
SIGMOID(X) = 1 / (1 + EXP(-X)).
DZDX = VL_NNSIGMOID(X, DZDY) computes the derivative of the block projected onto DZDY. DZDX and DZDY have the same dimensions as X and Y respectively.