[WIP] Wrapper for OT barycenter solvers with free support

[cedricvincentcuaz]

Types of changes

• creation of the class ot.utils.BaryResult extending OTResult

• Long term objectives: Wrap all barycenter solvers available in POT and more, ideally establishing links between these wrappers and those for inner OT solvers ot.solve_sample, ot.solve, and ot.solve_gromov - respectively associated with their barycenter solver ot.solve_bary_sample (free support); ot.solve_bary (free masses); ot.solve_bary_gromov (free joint support)

• a) ot.solvers.bary_free_support systematically optimizes the support of the barycenter using BCD with closed forms. Only metrics in ('sqeuclidean', 'euclidean') are supported for now. The solver calls an hidden function _bary_sample_bcd.

• b) handle variations in masses across OT problems only for unbalanced problems - keeping masses unchanged for exact and inexact sinkhorn like solvers - via parameter update_masses in ot.solvers._bary_sample_bcd.

• c) Handle given initialization of transport plans & potentials + warmstart strategies : added parameter warmstart to bary_sample, and based on the requested inner solver, we pick the adequate strategy warmstart_plan or warmstart_potentials passed to _bary_sample_bcd .

• d) Options for stopping criterion between losses and barycenter variations: added parameter stopping_criterion in ('loss', 'bary') passed to _bary_sample_bcd.

Extensions let for future works:

• a) extend metrics via the generalized free support barycenter solvers.
• b) include lazy inner solvers referenced in the list lst_method_lazy (in ot.solvers)

How has this been tested (if it applies)

• a) test.test_solvers.assert_allclose_bary_sol method to compare solutions as BaryResult objects.
• b) test_solve_bary_sample grid across all available barycenter solvers depending on those available in ot.solvers.solve_sample.
• c) test_solve_bary_sample_NotImplemented test error messages for methods not implemented yet.

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.

[codecov]

Codecov Report

❌ Patch coverage is 91.55556% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.82%. Comparing base (41a4d57) to head (05b47fc).

Additional details and impacted files

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- Coverage   96.87%   96.82%   -0.06%     
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  Files         113      113              
  Lines       23062    23285     +223     
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+ Hits        22342    22546     +204     
- Misses        720      739      +19     

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