Function einsum2_into 

pub fn einsum2_into<T: Scalar, OpA, OpB, ID: AxisId>(
    c: StridedViewMut<'_, T>,
    a: &StridedView<'_, T, OpA>,
    b: &StridedView<'_, T, OpB>,
    ic: &[ID],
    ia: &[ID],
    ib: &[ID],
    alpha: T,
    beta: T,
) -> Result<()>
where
    OpA: ElementOp<T> + 'static,
    OpB: ElementOp<T> + 'static,

Binary einsum contraction: C = alpha * contract(A, B) + beta * C.

ic, ia, ib are axis labels for C, A, B respectively. Axes are classified as:

• batch: in A, B, and C
• lo (left-output): in A and C, not B
• ro (right-output): in B and C, not A
• sum (contraction): in A and B, not C
• left_trace: only in A (summed out before contraction)
• right_trace: only in B (summed out before contraction)