[MRG] Fix swapped div_to_product arguments in fused_unbalanced_across_spaces_cost

[itzzdev09]

Types of changes

• Bug fix

Motivation and context / Related issue

Fixes #854.

div_to_product takes (pi, a, b, pi1=None, pi2=None, ...), but both calls in fused_unbalanced_across_spaces_cost (the reg_type="independent" / UCOOT branch) passed (pi, pi1, pi2, a, b, ...). The entropic terms were therefore computed with the plan marginals as the reference measures a, b and the reference measures px, py as the marginals pi1, pi2. For divergence="l2", where pi1/pi2 are ignored, the cost was taken against the product of the plan's own marginals instead of px ⊗ py.

This swaps the arguments back into the documented order for both the sample and the feature coupling.

How has this been tested (if it applies)

Added test_fused_unbalanced_across_spaces_cost_independent_reg (KL and L2): it checks that the regularisation part of the cost equals eps_samp * div_to_product(pi_samp, px_samp, py_samp) + eps_feat * div_to_product(pi_feat, px_feat, py_feat). The test fails on current master for both divergences and passes with the fix. test/gromov (217 passed) and test/test_ucoot.py (133 passed) pass locally with the numpy backend.

AI disclosure: this fix and test were made with Claude Code under my supervision; I checked the argument order against the div_to_product signature and ran the tests locally.

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

🤖 Generated with Claude Code

[6Ulm]

All good to me.

Nice catch! Just a small note: this bug concerns only the regularisation term. In practice, epsilon is 0 or very small, so this bug has limited impact on the final results (OT plans and costs).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.85%. Comparing base (3073aa0) to head (46b3042).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #855   +/-   ##
=======================================
  Coverage   96.85%   96.85%           
=======================================
  Files         128      128           
  Lines       26161    26181   +20     
=======================================
+ Hits        25337    25357   +20     
  Misses        824      824           

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