cuGraph-PyG Example Overhaul

[alexbarghi-nv]

The cuGraph-PyG examples need to be updated to be more consistent, performant, concise, and easier to understand.

1. The same code is reused for setup, termination, etc. across examples. This should be standardized into a common function that all examples use.
2. All examples should use PyTorch memory pools to ensure memory is properly shared between PyT and RMM.
3. All examples should support toggling managed memory on/off.
4. All examples (when applicable) should support running with directed/undirected versions of the graph via a command-line argument.
5. Examples should be annotated explaining what loaders/stores/models/encoders are doing.

[alexbarghi-nv]

Related: #140

[alexbarghi-nv]

Also Related: #17