Return Optional to TrainerHparams

[abhi-mosaic]

Leaving this as a draft as I'm not sure if this is intentional or not... but we seem to allow Optional args to the Trainer but not our TrainerHparams. Why is this?

Context: I would like to specify save_latest_artifact_name: null for WandB checkpointing as it is unnecessary and makes the UI cluttered. But TrainerHparams won't allow me to...