[MRG] add kl_loss to all semi-relaxed (f)gw solvers

[cedricvincentcuaz]

Types of changes

• Add loss_fun="kl_loss" to all semi-relaxed (f)gw solvers.
• line-search for conditional gradient solvers solve_semirelaxed_gromov_linesearch now support any decomposable functions as for gromov-wasserstein : implied to add a parameter fC2t (which corresponds to $f_2(C_2)^\top$ in this note). Previously existing parameters $C_1$ and $C_2$ should be past $h_1(C_1)$ and $h_2(C_2)$ as values - this is explained in the function documentation.

Motivation and context / Related issue

How has this been tested (if it applies)

• Extended most existing tests for these functions by looping over admissible values of loss_fun.

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

Merging #559 (c2e5d4d) into master (a73ad08) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #559   +/-   ##
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  Coverage   96.49%   96.49%           
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  Files          67       67           
  Lines       14663    14669    +6     
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+ Hits        14149    14155    +6     
  Misses        514      514