Validate loss_fun parameter in Gromov-Wasserstein utils

[kachayev]

Types of changes

ot.gromov._utils.init_matrix raises ValueError when unknown loss_fun is given.
ot.gromov._utils.init_matrix_semirelaxed raises NotImplementedError for loss_fun='kl_loss'.
ot.gromov._utils.init_matrix_semirelaxed raises ValueError for unknown loss_fun.

Motivation and context / Related issue

Error message is rather hard to understand when unsupported loss_fun is used (as the code makes an attempt to call undefined function).

How has this been tested (if it applies)

New test cases are provided.

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 #513 (3f87316) into master (20cc202) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   96.05%   96.06%           
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  Files          65       65           
  Lines       13679    13706   +27     
=======================================
+ Hits        13140    13167   +27     
  Misses        539      539           

[agramfort]

thx @kachayev