Function sparse_hessian_vec 

pub fn sparse_hessian_vec<F: Float + BtapeThreadLocal, const N: usize>(
    f: impl FnOnce(&[BReverse<F>]) -> BReverse<F>,
    x: &[F],
) -> (F, Vec<F>, SparsityPattern, Vec<F>)

Available on crate feature bytecode only.

Batched sparse Hessian: packs N colors per sweep using DualVec.

Like sparse_hessian but reduces sweeps from num_colors to ceil(num_colors / N).