Fix incorrect design for RunningAverage, Frequency, ModelCheckpoint, ...

[vfdev-5]

🐛 Bug description

Thanks to PRs on mypy annotation by @gruebel we discovered few classes where we have to redesign methods in order to keep consistency with their base classes.

• Frequency: Activate mypy in ignite.metrics #1391 (comment)
• RunningAverage: Activate mypy in ignite.metrics #1391 (comment)
• Checkpoint: to_save should not be Optional but it is due to ModelCheckpoint and current implementation.
  

  ignite/ignite/handlers/checkpoint.py

  Line 241 in ddcce01

  to_save: Optional[Mapping],

@gruebel please comment here if you think about other classes/methods we did a type relaxation.

cc @sdesrozis @fco-dv

[gruebel]

Similar to Frequency also GpuInfo inherits from Metric, so the attach method should have the same signature

ignite/ignite/contrib/metrics/gpu_info.py

Lines 107 to 108 in ddcce01

	def attach(self, engine: Engine, name: str = "gpu", event_name: Union[str, EventEnum] = Events.ITERATION_COMPLETED):
	engine.add_event_handler(event_name, self.completed, name)

Should be also type ignored in issue #1406 till it is fixed.

[vfdev-5]

Closing as solved