Convert SG and MG PyG Examples to Notebooks

[alexbarghi-nv]

We have been asked by the RAPIDS team to convert the PyG example scripts (graph_sage_sg.py and graph_sage_mg.py) to Jupyter notebooks in order to better align with user expectations.

[rlratzel]

Do we need to make sure these run and pass the CI notebook test runs, or are we going to use the "skip test" magic comment? If they need to run as tests (which seems like a good idea), then we'll need to make sure ci/test_notebooks.sh or more likely dependencies.yaml includes the right dependencies.

[rlratzel]

offline discussion: we agreed that running these new notebooks in CI would be a good idea.

[alexbarghi-nv]

Not planned; we are not converting the examples to notebooks.