Refactor losses

[vitusbenson]

The losses are currently a mess and need a better API.

Goal:

• Move dataset & model specific out of the losses
• More explicit naming
• Docstrings that indicate the maths behind each loss

Requirements:

• Need new abstraction that can handle seperation of loss computation from dataset/model specific stuff
• First version should support:
  • [ ] L1 loss
  • [ ] L2 loss
  • [ ] masking (both by data quality mask & by landcover mask)
  • [ ] Weight decay (as a regularization)

Culprits:

• Where to move the masking of preds & targets? Currently it is computed inside the loss function. However, it might be very dataset specific, so what is a good API for this?

Risks:

• Will likely break backward-compatibility