Various QOL changes

[mttk]

• Enable retrieving a Field from a Dataset by its name.
  • Motivation: we're using a built-in dataset (HF or Podium) and we want to get the Vocab of a specific Field. Currently, we have to rely on list ordering, now we can reference by name.
• Enable custom matrix type for batches
  • Now you don't have to cast every numpy array to torch, etc. You can pass a callable (which can also put the array on gpu etc).
  • Should look into making this a bit cleaner, e.g. via a decorator for iterators?
• Document dataset.split in rst
• Document dataset.concat in rst

[FilipBolt]

LGTM