Transform `y` into 2D tensor when `y.ndim == 1` and `likelihood == REGRESSION`

[wiseodd]

Fixes #235

Rationale on why we don't want to throw an error:

• PyTorch works on both these 1D-output cases:
  • model(x).squeeze() with y.shape == model(x).shape[0:-1]
  • model(x) with y.shape == model(x).shape
• So, in the wild, users' dataloaders vary between those to y.shape's. It's too much to ask to require y.shape == model(x).shape since PyTorch doesn't.