Generic Loader for OGB Node Pred datasets

[kaansancak]

It would be nice to have a PGLNodePropPredDataset function. OGBN-Arxiv dataset is already in place. The new functionality should just take an extra parameter (dataset name). This would be really useful for benchmarking with OGBN datasets.

I am aware that you already have PglNodePropPredDataset, however this lies within the legacy code and the module is not included in the new versions of pgl.

[Yelrose]

Thanks for your suggestion. It's a good idea to have a PglNodePropPredDataset. We are going to refactor our citation_benchmark into NodePropBenchmark.