Loading a subset of fields in NNPInput and Metadata

[ArnNag]

There are some situations where we do not need to load all the fields currently included in the BatchData instance returned by collate_conformers. For example, we do not need to load forces when training only on energies, need neither forces nor energies if we just want to run inference, do not need total charge for some of the models, etc. Should we subclass NNPInput and Metadata for different combinations of datafields?