Generalize the loss value returned from before_backward_pass callbacks (#38)

* Instead of a single additive value (which so far represented only the
  regularizer loss), callbacks return a new overall loss
* Policy callbacks also return the individual loss components used to
  calculate the new overall loss.
* Add boolean flag to the Scheduler's callback so applications can choose
  if they want to get individual loss components, or just the new overall
  loss
* In compress_classifier.py, log the individual loss components
* Add test for the loss-from-callback flow