Investigate Migrating cugraph.gnn Subpackage

[alexbarghi-nv]

cuGraph-PyG and cuGraph-DGL are being decoupled from cuGraph to rely on pylibcugraph instead, but we still have a cugraph.gnn subpackage within cugraph that both libraries need. We should find a better long-term home for that subpackage, especially since we are thinking of changing the release cycle of cuGraph-PyG and cuGraph-DGL and those packages are very tightly coupled with cugraph.gnn.

[alexbarghi-nv]

This is complicated by the distributed sampler's reliance on cuDF - maybe there could be an alternative where pytorch tensors are written instead so that cuDF could be an optional dependency.