Function left_environment 

pub fn left_environment<T: SVDScalar + EinsumScalar>(
    mpo_a: &MPO<T>,
    mpo_b: &MPO<T>,
    site: usize,
    cache: &mut Vec<Option<Matrix2<T>>>,
) -> Result<Matrix2<T>>
where
    <T as ComplexFloat>::Real: Into<f64>,

Compute the left environment at site i for MPO contraction

The left environment L[i] represents the contraction of all sites 0..i for the product of two MPOs A and B.

L[i] has shape (left_a_i, left_b_i) representing the accumulated contraction from the left.