Add Nightly Testing for cuGraph-PyG Examples

[alexbarghi-nv]

We have not had much luck getting these tested in CI, and have had to disable them. However, these are very important and often the best way to catch API breakages and edge cases. We also need to make sure they are tested on multiple GPUs - even the old testing in CI did not cover MG testing, which led to some issues being hidden.

Blocked by rapidsai/graph_dl#838