Function farneback_flow 

pub fn farneback_flow(
    prev: &Tensor,
    next: &Tensor,
    config: &FarnebackConfig,
) -> Result<(Tensor, Tensor), ImgProcError>

Farneback dense optical flow between two frames. Compute dense optical flow using Farneback’s polynomial expansion method.

prev and next are grayscale images [H, W] with values in [0, 1]. Returns (flow_x, flow_y) each of shape [H, W] containing per-pixel displacement.