[ENH] DummyRegressor for time series regression

[badrmarani]

Reference Issues/PRs

Fixes #3899.

What does this implement/fix? Explain your changes.

Adds a DummyRegressor that wraps over a sklearn DummyRegressor and inherits from BaseRegressor.

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

No.

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], [MNT], [DOC], or [BUG] indicating whether the PR topic is related to enhancement, maintenance, documentation, or bug.

[fkiraly]

Nice! I've started the CI.

[fkiraly]

linting is failing, kindly ensure you format your code correctly.
Guideline here: https://www.sktime.org/en/stable/developer_guide/coding_standards.html

[fkiraly]

(see above)

[badrmarani]

Hi,

I've gone ahead and fixed the linting issues on my last pull request. I believe everything should be good to go now.

Thank you!

[fkiraly]

Tests are now genuinely failing. I think this may be one or both fo the following reasons:

• regressors should return floats in predict, and expect float y in fit - not strings
• when you compare equality with np.all, you might be missing a list comprehension

[fkiraly]

thanks a lot, useful!

there are some small fiddly issues to sort out, see the failing tests

[badrmarani]

I've gone ahead and fixed the unit test related errors. Code should now be able to pass the tests.

[fkiraly]

Nice, they pass now! Thank you!

[fkiraly]

Great!

I made a minor change: put the docs section in alphabetical order