Enables Successive Halving search-estimators

The API and results of these estimators might change without any deprecation cycle.

Importing this file dynamically sets the HalvingRandomSearchCV and HalvingGridSearchCV as attributes of the model_selection module:

>>> # explicitly require this experimental feature
>>> from sklearn.experimental import enable_halving_search_cv # noqa
>>> # now you can import normally from model_selection
>>> from sklearn.model_selection import HalvingRandomSearchCV
>>> from sklearn.model_selection import HalvingGridSearchCV

The # noqa comment comment can be removed: it just tells linters like flake8 to ignore the import, which appears as unused.