First major release

First Major Release

This is the first major release of OGB.
A number of changes have been made to the datasets, which are summarized below.

1. Re-indexed all the nodes in the node/link datasets (The graphs remain essentially the same).
2. In dataset folders for all the datasets, added mapping/ directory that contains information to map node/edge/graph/label indices to real-world entities (e.g., mapping from nodes in PPA to unique protein identifiers, mapping from molecular graphs into the SMILES strings.)
3. Deleted the ogbn-proteins node features, and put them in the species variable.
4. Deleted ogbl-reviews datasets.
5. Added 4 datasets: ogbn-arxiv, ogbl-citation, ogbl-collab, ogbl-wikikg.
6. Renamed ogbg-ppi to ogbg-ppa.
7. Renamed ogbg-mol-hiv and ogbg-mol-pcba to ogbg-molhiv and ogbg-molpcba, respectively.
8. Changed the evaluation metric of imbalanced molecule dataset (e.g., pcba) from ROC-AUC to PRC-AUC.
9. Changed the get_split_edge() interface in LinkPropPredDataset. The downloaded dataset files are also changed accordingly.
10. Added num_classes attribute for multi-class classification datasets.