FIX Uses log2 in tree building

[thomasjpfan]

Reference Issues/PRs

Fixes #30554

What does this implement/fix? Explain your changes.

This PR replaces log with log2 and explicitly uses that in sklearn/tree/_partitioner.pyx, so it's more clear that we are using base 2.

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[OmarManzoor]

LGTM. Thanks @thomasjpfan

[jeremiedbb]

The name of the fragment should be 30557.fix.rst. Otherwise LGTM

[glemaitre]

Do you think that it is worth a minimal non-regression test to acknowledge that we use the log2?

[ogrisel]

We might also want to delete the log function of https://github.com/scikit-learn/scikit-learn/blob/6385b7f7f4395f050cb4c85449fa3987031598c1/sklearn/tree/_utils.pyx#L65C1-L66C27 and update any remaining calls such as sklearn/tree/_criterion.pyx to cimport log2 from libc.math instead.

This change would better be done in another MAINT PR that would not be part of the 1.6.1 released fix.

[ogrisel]

Do you think that it is worth a minimal non-regression test to acknowledge that we use the log2?

I approved the PR but forgot about this point. I am not sure how to do that, but it would be great to find a way.

[thomasjpfan]

I added a non-regression test that uses an example that fails on main and passes with this PR.

[thomasjpfan]

I added this function to test sort directly.