Implement ZINC Evaluation Metrics

[caio-freitas]

The evaluation metrics for the ZINC dataset are still to be implemented in this project.

The repository https://github.com/cvignac/DiGress/tree/main/src/metrics contains implementations of multiple metrics for molecule predictions, such as

• Validity
• Uniqueness
• NSPDK (Neighborhood Subgraph Pairwise Distance Kernel)
• FCD (Frechet ChemNet Distance)