FIX Fixes bug OneHotEncoder's drop_idx_ when there are infrequent categories

[thomasjpfan]

Reference Issues/PRs

Fixes #25550

What does this implement/fix? Explain your changes.

This PR adds a _drop_idx_internal to OneHotEncoder that is used to drop the categories. _drop_idx_internal was already precomputed to take into account the grouped infrequent categories.

The public drop_idx_ attribute needs to be remapped to reference back to the category that was actually dropped. There are tests in this PR to assert this behavior.

Any other comments?

I was not able to think of a simpler way to do this without adding another attribute.

[jeremiedbb]

I'm not very familiar with all the one hot encorder internals, so I'm having a hard time reviewing the implementation :)
But the test seems to capture the behavior we expect.

I think it would be worth documenting more precisely the interaction between drop and infrequent categories. i.e. that infrequent categories are first grouped and then drop is resolved. In particular, "if_binary" applies to the categories after they've been grouped. This might not be very intuitive.

Other than that I can't see a way around having a separate attribute either. So I'd say LGTM.

ping @glemaitre or @ogrisel who might be more familiar with the internals of ohe

[glemaitre]

LGTM. I don't see a more straightforward way to handle the remapping. I propose only minor changes.

Since we are discussing about a remapping, just wondering if a private dict together with a property could make allow to store of a single attribute. But I did not think if it was actually feasible.

[jjerphan]

LGTM, thank you @thomasjpfan.

Here are just two nitpicks.