pub fn gradients_grayscale(
    image: &GrayImage,
    horizontal_kernel: Kernel<i32>,
    vertical_kernel: Kernel<i32>,
) -> Image<Luma<u16>> {
    gradients(image, horizontal_kernel, vertical_kernel, |p| p)
}
A special version of [`gradients`] function for grayscale images which doesn't require giving a
pixel mapping function.
src/gradients.rs