[MRG] inits for sr(F)GW + srGW barycenter + exemples

[cedricvincentcuaz]

Types of changes

• add initialization heuristics for sr(F)GW problems semirelaxed_init_plan in ot.gromov._utils.py.
• Integrate these initializations as options in sr(F)GW matching solvers by allowing G0 to also be a string among admissible init methods.
• add semirelaxed_gromov_barycenters following API of gromov_barycenters, while adding G0 : str to leverage init methods in semirelaxed_init_plan to both init transport plans and the barycenter.
• Integrate these inits in srFGW barycenter solver too.
• Update the lp_solver of semirelaxed_cg in ot.optim : relaxing the identification of the minimum in the gradient with a small margin to take into account precision errors. This was causing issues of consistency across backends.
• remove some useless kwargs in entropic sr(F)GW solvers, which were absorbing tol_rel and tol_abs parameters used in the solver ot.solve_gromov, whereas only parameter tol is used in entropic sr(F)GW solvers.
• add exemple for both srGW and srFGW barycenters for dictionary learning.

Motivation and context / Related issue

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

Attention: Patch coverage is 99.59514% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.85%. Comparing base (0b59002) to head (0612031).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #662      +/-   ##
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+ Coverage   96.78%   96.85%   +0.06%     
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  Files          88       88              
  Lines       17506    17910     +404     
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+ Hits        16944    17346     +402     
- Misses        562      564       +2     

[rflamary]

Thanks @cedricvincentcuaz , a few comments

[cedricvincentcuaz]

Thank you for your review @rflamary, I've finished taking your comments into account.