BUG Add tol to _make_unique to avoid inf values in IsotonicRegression

[lucyleeow]

Reference Issues/PRs

Fixes #10903
Closes #10981 and #17758

What does this implement/fix? Explain your changes.

Add a tol when checking equality in _make_unique, such that values are deemed unique only if their difference is greater or equal than 1e-15 i.e.:

if x - current_x >= 1e-15:

this agrees with Python:

>>> 1 == 1+1e-15
False
>>> 1 == 1+1e-16
True

Note that this is not the case for 0:

>>> 0 == 1e-15
False
>>> 0 == 1e-323
False
>>> 0 == 1e-324
True

see more here: #17758 (comment)

Any other comments?

Also removed current_count as it does not seem to be used anymore (as discussed with @ogrisel)
cc @ogrisel

[ogrisel]

Could you please add a new non-regression test for IsotonicRegression based on #17758 (comment) as part of this PR to check that this fixes the source of the original issue?

[ogrisel]

Thanks for fixing this nasty bug @lucyleeow! Here are a few more suggestions for improvements. But otherwise LGTM.

[ogrisel]

FYI, I merged the master branch that has a fix for the unrelated failing macOS CI.

[lucyleeow]

Thanks for the suggestions @ogrisel, added!

[ogrisel]

This looks good to me! Can you please add a new entry for doc/whats_new/0.24.rst?

[lucyleeow]

Maybe 2nd review @glemaitre 😄 ?

[agramfort]

thanks @lucyleeow !