[FEA] Migrate All PyTorch-Dependent Code to cuGraph-GNN

[alexbarghi-nv]

We continue to have issues with PyTorch in this repository's CI. In addition, the code that depends on PyTorch is really only used by cuGraph-GNN and does not really fit the mission of this repository. As discussed over the past few months, we want to migrate as much GNN code as possible to the cugraph-gnn repository.

There are three key pieces of code affected:

1. The FeatureStore class which is about to be deprecated (in release 25.02)
2. The BulkSampler class which is also about to be deprecated (in release 25.02)
3. The DistSampler class, the replacement for BulkSampler, which is a fundamental piece of our GNN infrastructure.

There is also going to be some additional code in the very near future supporting GNN use cases related to GraphRAG, graph databases, and other frameworks (beyond DGL and PyG). This would also better fit within cugraph-gnn.

We propose creating a new package, pylibcugraphgnn, which will contain the bulk sampling code, as well as any other framework-agnostic code and/or thin wrappers around our C++ code for GNN operations. This package will presumably launch with release 25.04.

[rlratzel]

Would pylibcugraph-gnn be used by any other package beyond cugraph-gnn? For example, we created pylibcugraph so the same Cython bindings could be used by multiple higher-level APIs (cugraph, nx-cugraph, cugraph-pyg, etc.). If the PLC-gnn code is only (and always?) used by cugraph-gnn, can it just be part of the same package? I'm not seeing the advantage of another package if it's only used by cugraph-gnn (and presumably not optional), or maybe I'm missing something?

[alexbarghi-nv]

Right now it's used by cuGraph-PyG and cuGraph-DGL. The long-term goal is to integrate with other frameworks, including pure PyTorch. But I suppose we might still be able to support that within cuGraph-PyG. So then only cuGraph-DGL would be the other dependency, and we have been debating what to do about that package.

[alexbarghi-nv]

Also there is no cugraph-gnn package. Just cugraph-pyg, cugraph-dgl, pylibwholegraph, and libwholegraph.