Internal callback changes

[lorenzoh]

This PR includes internal changes to callback and callback state:

• Scheduler and Metrics store step state separately for each phase. As a result TrainingPhase is no longer "special".
  • learner.cbstate.metricsstep is now a dict of Phase => MVHistory instead of just one mvhistory.
  • learner.cbstate.history is now a dict of Phase => History instead of just one history.
• Added utilities for adding, removing or changing callbacks:
  • addcallback!(learner, cb)
  • getcallback(learner, C)
  • replacecallback!(learner, cb)
  • removecallback(learner, C)
• The Init handler of included callbacks is now idempotent, allowing it to be retriggered when callbacks are changed.

It also includes SmoothLoss(beta) a metric of the loss with exponential weight.

[Moelf]

SmoothLoss has no example not tests