Function autodiff_unflatten_pair 

pub fn autodiff_unflatten_pair<B: Backend, C: CheckpointStrategy, const DA: usize, const DB: usize>(
    combined: FloatTensor<Autodiff<B, C>>,
    flat_y_len: usize,
    flat_s_len: usize,
    shape_y: [usize; DA],
    shape_s: [usize; DB],
) -> (FloatTensor<Autodiff<B, C>>, FloatTensor<Autodiff<B, C>>)

Inverse of flatten_pair: split a 1-D combined tensor back into the two outputs at their original ranks/shapes.