public void SigmoidGradient()
        {
            var engine  = new NeuralNetworkEngine(_config400x25x10);
            var sigmoid = engine.SigmoidGradient(new DenseVector(new double[] { 0 }));

            sigmoid.Should().NotBeNull().
            And.HaveCount(1);
            sigmoid[0].Should().Be(0.25);
        }