Function contract_fit 

pub fn contract_fit<T: SVDScalar + EinsumScalar>(
    mpo_a: &MPO<T>,
    mpo_b: &MPO<T>,
    options: &FitOptions,
    initial: Option<MPO<T>>,
) -> Result<MPO<T>>
where
    <T as ComplexFloat>::Real: Into<f64>,

Perform variational fitting contraction of two MPOs

This computes C = A * B using a variational (DMRG-like) algorithm that alternates between sweeping left-to-right and right-to-left, optimizing two sites at a time.

Arguments

• mpo_a - First MPO
• mpo_b - Second MPO
• options - Fitting options
• initial - Optional initial guess (if None, uses naive contraction)

Returns

The contracted MPO C with bond dimension controlled by options