Dgllife

This release:

- includes various pre-trained models for 12 datasets in MoleculeNet. See [here](https://github.com/awslabs/dgl-lifesci/tree/master/examples/property_prediction/moleculenet)
- fixes two bugs related to `MoleculeCSVDataset` and the WLN example for reaction prediction.
- supports two kinds of scaffold split, one based on `MurckoDecompose`, the other based on `MurckoScaffoldSmiles`

This release **supports DGL 0.5.0+** along with some feature enhancement. Note that it is recommended to use **DGL 0.5.2+**, which fixes some bugs. The feature enhancement includes:
- 67 Support for parallel graph construction in `MoleculeCSVDataset` (kudos to sooheon )
- Better support for molecular property prediction on custom CSV datasets, which can be found [here](https://github.com/awslabs/dgl-lifesci/tree/master/examples/property_prediction/csv_data_configuration)
- 65 66 Bug fix for inference with fine-tuned GINs on custom CSV datasets (kudos to skrsna )
- 73 Support for self loops in bond featurization
- 74 Support for virtual nodes in graph construction and featurization
- 75 Bug fix for featurizing self loops in graphs without real edges
- 61 81 82 83 86 Built-in support for more datasets from MoleculeNet
- 79 80 Support for PR-AUC Metric
- 87 Filter out invalid molecules for `MoleculeCSVDataset`
- 84 85 Support non-ring systems in scaffold split (kudos to skrsna )