code: public SDVariable sigmoidDerivative(SDVariable x, SDVariable wrt) {
          return sigmoidDerivative(null, x, wrt);
      }
docs: Element-wise sigmoid function derivative: dL/dIn given input and dL/dOut
  
  @param x   Input Variable
  @param wrt Gradient at the output - dL/dOut. Must have same shape as the input
  @return Output variable