Reference Issues/PRs

Towards #23958
Supersedes #25561

What does this implement/fix? Explain your changes.

From #25561:
This simplifies the original implementation of PairwiseDistance by @jjerphan, with the following differences:

• PairwiseDistance{32,64} doesn't subclass BaseDistancesReduction{32,64} anymore.
• This allows to add _parallel_on_{X,Y} methods to PairwiseDistance{32,64}, since these methods are decorated with @final in BaseBaseDistancesReduction{32,64} and thus can't be overwritten.
• This also remove the chunk computing mechanism, by considering only the case chunk_size = 1, as proposed by @ogrisel in this comment.
• This doesn't implement the Euclidean specialization yet to make benchmarks simpler.

This PR:

• Preserves existing specializations and dispatches based on benchmark-driven heuristic (benchmarks will be published soon)
• Simplifies usability criteria for PairwiseDistances
• Changes backend dispatch process in pairwise_distances
• Adds n_jobs parameter to PairwiseDistances to preserve n_jobs semantics from pairwise_distances
• Moves X_is_Y attribute to PairwiseDistances to minimize change to unrelated API (all that would be affected by DatasetsPair)
• Cleans misc. changes/comments and updates PR
• Minimizes diff

Any other comments?

Benchmarks will be coming soon