Ensure periodic saving fires immediately after runner task is finished

[jbweston]

Description

This PR modifies the behavior of periodic saving to fire immediately after the runner completes.
Previously the periodic saving would wait 'interval' seconds before firing the final save event.

Fixes #439 .

Checklist

• Fixed style issues using pre-commit run --all (first install using pip install pre-commit)
• pytest passed

Type of change

Check relevant option(s).

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• (Code) style fix or documentation update
• This change requires a documentation update

[akhmerov]

Cool implementation!

What will this do if the runner.task is canceled? Do I understand correctly that it'll still save one last time?

[jbweston]

Cool implementation!

What will this do if the runner.task is canceled? Do I understand correctly that it'll still save one last time?

It will indeed save one last time.

I believe this to be the semantics we want; we want all the results that the learner has at a particular point in time to be reflected in the saved results, regardless of whether the runner was cancelled, or stopped of its own accord.