Implement transforms as dictionaries

[raghavmecheri]

Feature Request

Description of Problem:

Right now, we just pass a single transform object in. This is inconvenient if we want different transforms for train, test, and val, as mentioned in #25.

Potential Solutions:

Rename the transform parameter to transforms and treat it as a dictionary instead. We can then split it within fetch_segmented_dataloaders . We would also want to update our tests to reflect this change + cover the edge case where the transforms parameter is either None or {}