Refactored model ckpting and loading

[rampasek]

Refactored model checkpointing, loading, and running inference using a pre-trained model:

• Added option to checkpoint the best model based on validation performance instead of fixed frequency schedule
• Minor refactor for loading a pre-trained model
• Support for loading an existing model to run just inference
• Example cfgs for ZINC to save the best model and then to load it for inference