Add callback.run_event()

[ajaysaini725]

Add a helper method in callback for run_event. This helper method would then call the correct method on callback. It would look something like:

class Callback:
  def run_event(self, state; State, logger; Logger, event: Event):
    if event == Event.TRAINING_START:
        self.training_start(state, logger)
    if event == Event.BEFORE_FORWARD:
        self.before_forward(state, logger)
    ...

Then, the engine would do callback.run_event(state, logger, event).

This would help clean up code in the following places:

• RankZeroCallback: Instead of monkeypatching each callback function, it would simply override run_event.
• RankZeroLogger: No need for a private _training_start method that is different from all of the other callbacks
• Checkpointing tests: The EventCounterCallback basically does this, via monkeypatching