[WIP] Wrappers for OT barycenter solvers

[cedricvincentcuaz]

Types of changes

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

first draft for OT barycenter solvers with free support and predefined metric ot.solvers.bary_free_support built on top of ot.solvers.solve_sample, extending significantly available OT objectives for the barycenter problem.

• 1-a) ot.solvers.bary_free_support systematically optimize the support of the barycenter using BCD with closed forms, and optionally learn masses if not learn_b=False. Only metrics in ('sqeuclidean', 'euclidean') are support for now. The solver calls an hidden function _bary_sample_bcd.
• 1-b) [TO Check] extend metrics via the generalized free support barycenter solvers.
• 1-c) 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.
• 1-d) 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 .
• 1-e) Options for stopping criterion between losses and barycenter variations: added parameter stopping_criterion in ('loss', 'bary') passed to _bary_sample_bcd.
• 2. [TO DO] see how to handle best learn_b=True
• 3. [TO DO] Extension to support lazy tensors as in ot.solve_sample
• 5. [TO DO] Complete documentation

Motivation and context / Related issue

• 1. test.test_solvers.assert_allclose_bary_sol method to compare solutions as BaryResult objects. test_bary_sample grid across all available barycenter solvers depending on those available in ot.solvers.solve_sample.

How has this been tested (if it applies)

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 87.92271% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.06%. Comparing base (5a7d086) to head (7132f62).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #730      +/-   ##
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- Coverage   97.15%   97.06%   -0.09%     
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  Files         107      107              
  Lines       21877    22082     +205     
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+ Hits        21254    21434     +180     
- Misses        623      648      +25     

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