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This release contains breaking changes:
* Renamed cb_phase to apply_on_phase
* Renamed CallbackPhase to Phase, TrainerState to State
* Renamed Trainer.stop_training() to Trainer.stop(), so it also apply to testing
* Trainer now keeps track of iteration count and iteration_in_epoch count
* CallbackContext was expanded with iteration count and iteration_in_epoch count
* Calling Trainer.stop() from a callback will now stop even on a batch level (and not only epoch level)