Types of changes

• Add exact line-search for ot.gromov.gromov_wasserstein and ot.gromov.fused_gromov_wasserstein when loss_fun = 'kl_loss' which can be called by setting armijo=False(default for the method). Note that these solvers ignored the provided armijo parameter with this loss enforcing armijo=True.
• Extend ot.gromov.solve_gromov_linesearch to any loss with common decomposition.
• Adapt the behavior of dependent solvers that also enforced armijo=True: ot.gromov.gromov_barycenters and ot.gromov.fgw_barycenters
• Add gradients in this setting for ot.gromov.gromov_wasserstein2 and ot.gromov.fused_gromov_wasserstein2

Motivation and context / Related issue

How has this been tested (if it applies)

• Extended existing test for ot.gromov.gromov_wasserstein and ot.gromov.fused_gromov_wasserstein with kl loss, looping over armijo values.

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 Report

Merging #556 (e3f35cd) into master (53dde7a) will not change coverage.
The diff coverage is 100.00%.

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##           master     #556   +/-   ##
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  Coverage   96.49%   96.49%           
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  Files          67       67           
  Lines       14663    14663           
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  Hits        14149    14149           
  Misses        514      514