Make base_folder configurable and add attribute handling for DDP vs non-DDP UNet model#60

This PR includes the following changes: 1. Instead of hardcoding `base_folder` (where data is located), make it configurable via command line argument. 2. Add attribute handling to reassign `unet` variable properly when using DDP. An extra `.module` variable is populated in `model` object when using DDP, but for users not using DDP, the code as it is currently written will crash. To fix this, we simply add conditional logic which sets `unet = unet.module` if DDP is being used, otherwise do nothing.