[MRG] Fix segfault in ot.emd when no cost is finite

[itzzdev09]

Types of changes

• Bug fix

Description

ot.emd and ot.emd2 crash the Python interpreter with a segmentation fault when every entry of the cost matrix is NaN or infinite:

import numpy as np, ot
a, b = ot.unif(4), ot.unif(3)
ot.emd(a, b, np.full((4, 3), np.nan))   # Segmentation fault
ot.emd(a, b, np.full((4, 3), np.inf))   # Segmentation fault

Everything built on them crashes too, e.g. ot.solve, and ot.gromov.gromov_wasserstein2 with an all-NaN structure matrix. That is the case #469 reported; it was closed because a single NaN now gives a NaN result, but a fully NaN matrix still crashes.

Only the all-non-finite case crashes. With a NaN or inf row, column, block, or every entry but one, the solver returns an infeasible result (zero plan, result_code 0, the usual "Problem infeasible" warning). The dense solver call now goes through a small _emd_c_dense helper that returns that same infeasible result when no cost is finite, without calling emd_c. Every other input behaves exactly as before. With this, gromov_wasserstein2 on an all-NaN structure returns NaN, as for a single NaN.

How has this been tested

Added test_emd_no_finite_cost (NaN and inf) in test/test_ot.py. It covers emd, emd2 and gromov_wasserstein2, and it segfaults on master. test_ot.py, test_1d_solver.py, test_solvers.py and test/gromov pass (593 passed).

Checklist

• I have added tests to cover my changes.
• I have updated the RELEASES.md file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.86%. Comparing base (98d09a1) to head (3a301c1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #869   +/-   ##
=======================================
  Coverage   96.86%   96.86%           
=======================================
  Files         128      128           
  Lines       26304    26324   +20     
=======================================
+ Hits        25480    25500   +20     
  Misses        824      824           

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