Function associate_from_to 

pub fn associate_from_to(
    from_exprs: &[PartitionExpr],
    to_exprs: &[PartitionExpr],
) -> Result<Vec<Vec<usize>>>

Associates each expression in from_exprs with indices of overlapping expressions in to_exprs.

Output vector length equals from_exprs.len(), and each inner vector contains indices into to_exprs that overlap with the corresponding from_exprs[i].