Fix incorrect design of handlers (Frequency, RunningAverage, etc.)

[sdesrozis]

Fixes #1405

Description:

• Frequency : introduction of a specific usage FrequencyWise to fit the Metric.attach() api.

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)

[vfdev-5]

Looks good, thanks @sdesrozis !

[vfdev-5]

@sdesrozis thanks for the update! I was thinking as Checkpoint and ModelCheckpoint fix is the most simple and we do not break other things, it would be better to do it in a separate PR. For others, I'm not yet sure about the fix to adopt.

[sdesrozis]

Ok so if I understand correctly, I do a new PR with last commits about ModelCheckpoint fix, right ? And let's see for others ?

[vfdev-5]

Exactly!

[vfdev-5]

To be honest currently I'm not that sure about introducing things in this way. Let's try to find a better design that you already told previously about... I'd prefer to keep things as is instead of introducing a half-good solution. What do you think ?

[sdesrozis]

Not relevant at the moment. I prefer close this PR.