Improve RunningAverage reset when epoch_bound=False

[sadra-barikbin]

Description:
When epoch_bound=False, an explicit metric.reset() was required before
another engine.run. This PR gets RunningAverage to reset itself automatically.

Check list:

• New tests are added (if a new feature is added)

[vfdev-5]

Thanks for the PR @sadra-barikbin ! I left few comments and please update PR's description as it states :

This PR removes that requirement ...

but you add more code, this looks a bit misleading

[vfdev-5]

by the way, let's simplify this code as well:

-    @trainer.on(Events.STARTED)
-    def running_avg_output_init(engine):
-        engine.state.running_avg_output = None
+   trainer.state.running_avg_output = None

[vfdev-5]

Thanks, lgtm!