Removed warning in DDP if Metric.reset/update are not decorated.

[vfdev-5]

Description:

• Removed warning in DDP if Metric.reset/update are not decorated.

Context:
reinit__is_reduced and sync_all_reduce decorators are optional helpers to compute full metric value in distributed context. They should be optional and users basically are free to skip them and perform all_reduce-like ops on accumulated variable during compute on their own. And also make sure that compute wont perform collective ops twice if no reset/update were called. reinit__is_reduced decorator is needed to avoid multiple reductions if compute method is called several times without reseting or updating...
If user develop their own metric and prefer to all_reduce with their means, they see a warning in DDP.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)