🐛 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