settle on dataloader specs

[dkimpara]

We should have it output (B, C, t, lat, lon) and modify all models to be compatible.

e.g. one issue is we need to move concat_and_reshape and split_and_reshape in [model].py files to dataloader.

[dkimpara]

The old models (unet, rvt, vit etc.) are written for (Batch, color_channel, lat, lon).

We need to rewrite the old models to be compatible with the new data shape, with time=1. Charlie suggested we squeeze time on the way in and then expand on the way out.