[ENH] Extract cached ForecastingHorizon methods to functions and avoid B019 error

[khrapovs]

Reference Issues/PRs

closes #2338

What does this implement/fix? Explain your changes.

Extract ForecastingHorizon.to_relative and ForecastingHorizon.to_absolute to _to_relative and _to_absolute, respectively, to allow caching without B019 error.

Does your contribution introduce a new dependency? If yes, which one?

No

What should a reviewer concentrate their feedback on?

Any other comments?

PR checklist

For all contributions

• I've added myself to the list of contributors.
• Optionally, I've updated sktime's CODEOWNERS to receive notifications about future changes to these files.
• I've added unit tests and made sure they pass locally.
• The PR title starts with either [ENH], [DOC] or [BUG] indicating wether the PR topic is related to enhancement, documentation or bug

[fkiraly]

Looks good, thanks!

Non-blocking comments:

• might be worth to add to the comments preceding the new, losse functions, a refrence to the B019, or someone might get the idea to put them back
• you remember the pandas bug? I think we should maybe change the comments there from "replace when fixed" to "replace when we upgrade our lower pandas bound to a version where this is fixed"? Or we´ll have the bug back on the lower end of our supported versions...

[khrapovs]

@fkiraly Addressed both of your suggestions.

[fkiraly]

@fkiraly Addressed both of your suggestions.

Thanks!

May I ask for reformulation of this:

This function is a direct replacement of the ForecastingHorizon.to_relative method to avoid B019 error from flake8-bugbear.

The problem with this is that it's obsccure.
You need to imagine someone who has not been part of our conversation.
Hence they will not get the reference "replacement" which is referring to the earlier state, since they will see only the current state (in the future).

Would suggest to change to

"This function needs to be outside ForecastingHorizon since the lru_cache decorator has known, problematic interactions with object methods, see B019 error of flake8-bugbear for a detail explanation."

[khrapovs]

@fkiraly I change the wording as you proposed

[fkiraly]

readthedocs is fine (was before), but failing since too many readthedocs jobs are running concurrently