Coarsening symmetric graphs leads to slightly asymmetric edge weights

[seunghwak]

If you coarsen a symmetric (i.e. undirected) graph, the output graph should be symmetric as well.

However, due to limited floating point resolution, edge weights can be slightly asymmetric after coarsening (e.g. for a triplet of src, dst, weight, we may see (1, 2, 1.0) and its reverse edge (2, 1, 1.0 + 1e-7), this is only approximately symmetric and not strictly symmetric).

This PR fixes this by coarsening using only the lower triangular part (including the diagonal) after relabeling and reconstructing a symmetric graph from the lower triangular part (if the input graph is symmetric).

[codecov-commenter]

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[BradReesWork]

@gpucibot merge