Add `n_features_in_` attributes

[TomAugspurger]

We need to add an n_features_in_ attribute to our estimators.

https://github.com/scikit-learn/scikit-learn/pull/16112 / https://scikit-learn-enhancement-proposals.readthedocs.io/en/latest/slep010/proposal.html

https://dev.azure.com/dask-dev/dask/_build/results?buildId=1080&view=logs&j=d699f7f7-fbb1-5fe7-7f49-16a980a27dcf&t=47702d4b-eb38-52c1-7644-7bfb10716412

[TomAugspurger]

Added to some classes in #669.

[TomAugspurger]

Looks like CI has started failing: https://dev.azure.com/dask-dev/dask/_build/results?buildId=2095&view=results