MoleculeNet Classification Data : Modularization, dynamic splits, Clean Code

[aditya0by0]

Changes related to Molecule Net classification introduced in #130

• Remove Redundant Code logic for molecule net classification
• Modularize the molecule net classification code
• Enable dynamic splitting functionality
• Use deepchem to access data (https://deepchem.readthedocs.io/en/latest/api_reference/moleculenet.html)
• Use the recommended split type ("scaffold" or "random") for underlying data specified by https://arxiv.org/abs/1703.00564
• Disable split data size specification (we shouldn't use custom split size for these data rather we should follow the standards used in community which is 80/10/10, for fair comparison)
• Add Two more missing data classes PCBA and ToxCast for classification
• Delete Tox21 data class and test, instead new modularized class is created

General Changes

• Check before dataloader to assert if the data is empty (as pytorch or lightning doesn't check this)

• Generalize _DynamicDataset class to all datasets by replacing _graph_to_raw_dataset to _preprocess_data_into_dataframe

• Modularize splitting logic into separate classes (so we dont need to specifiy the split logic for every new data class, rather just inherent the required splitting logic class )

Suggested Metric as per data type by https://arxiv.org/abs/1703.00564 for Molecule Net dataset

Dataset	Task Type	Classes	Recommended Metric	Why
BACE	Binary classification	2	ROC-AUC	Standard benchmark metric; moderate class imbalance
BBBP	Binary classification	2	ROC-AUC	MoleculeNet standard; threshold-independent
HIV	Binary classification	2	ROC-AUC	Highly imbalanced dataset
MUV	Multi-task binary (17 tasks)	2	PR-AUC (Average Precision)	Extremely imbalanced; ROC-AUC can be misleading
PCBA	Multi-task binary (128 tasks)	2	PR-AUC (Average Precision)	Very sparse positives; PR-AUC is preferred
SIDER	Multi-task binary (27 tasks)	2	ROC-AUC	Standard MoleculeNet evaluation
ClinTox	Multi-task binary (2 tasks)	2	ROC-AUC	Standard benchmark metric
Tox21	Multi-task binary (12 tasks)	2	ROC-AUC	Widely used benchmark metric
ToxCast	Multi-task binary (hundreds of tasks)	2	ROC-AUC (sometimes PR-AUC as secondary)	Standard MoleculeNet metric

[aditya0by0]

@sfluegel05, once I complete the gnn experiments with molecule net dataset, then we could merge the PR. Until then we could keep this PR in the draft state.