[ENH] Implement series_to_series forecaster mixin

[rwtarpit]

Reference Issues/PRs

Fixes #2910

What does this implement/fix? Explain your changes.

This PR implements the Mixin class for series_to_series forecasters by introducing a abstract method _series_to_series_forecast.
A dummy series forecaster is also implemented in _dummy_series_forecaster.py for testing purpose.

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

Any other comments?

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I have added the following labels to this PR based on the title: [ enhancement ].
I have added the following labels to this PR based on the changes made: [ forecasting ]. Feel free to change these if they do not properly represent the PR.

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[rwtarpit]

@hadifawaz1999 this PR is regarding the unimplemented SeriesToSeriesForecastingMixin we talked about on slack.
pls suggest any changes

[hadifawaz1999]

Thanks for taking care of this. Few comments before going into details:

• Dummy forecaster should be under forecasring/deep_learning and should inherit base deep forecaster instead of base forecaster

[rwtarpit]

@hadifawaz1999 the last few tests that are failing are in python 3.13 enviroment specifically, due to tensorflow compatibility issues. Tensorflow is being imported in BaseDeepForecaster.
should i skip tests with tensorflow dependency?

[hadifawaz1999]

@hadifawaz1999 the last few tests that are failing are in python 3.13 enviroment specifically, due to tensorflow compatibility issues. Tensorflow is being imported in BaseDeepForecaster. should i skip tests with tensorflow dependency?

Hello, for all tests you should add this decorator:

@pytest.mark.skipif(
    not _check_soft_dependencies("tensorflow", severity="none"),
    reason="skip test if required soft dependency not available",
)