Restrict saving of validation .csv's to wandb

[AUdaltsova]

We currently have validation .csvs created and saved to wandb here and are quickly running out of storage space on wandb, because these files are saved each epoch for each run, and quickly become massive with horizon and quantile number increases. We should make these optional and cut down on the size/amount of files saved:

• only save best epoch
• only save the same 4(?) batches we save val plots for
• any other way we can trim this?