cugraph python graph symmetrization is prohibitively inefficient, needs optimization

[rlratzel]

Who: cugraph users What: improve memory efficiency when creating undirected graphs to reduce OOM errors Why: undirected graphs require a symmetrization step which is currently prohibitively inefficient, resulting in OOM errors where they seemingly should not. This results in users not being able to use cugraph when they, in many cases, should be able to based on input size and GPU memory size.