[WIP] Partial Entropic FGW solvers

[cedricvincentcuaz]

Types of changes

• Implement Entropic partial FGW solvers using projected gradient descent
• Add these solvers to ot.solve_gromov
• [TO DO] new exemples with (entropic) Partial FGW

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 98.15951% with 3 lines in your changes missing coverage. Please review.

Project coverage is 97.07%. Comparing base (b5cfb91) to head (4c97177).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #702      +/-   ##
==========================================
+ Coverage   97.05%   97.07%   +0.02%     
==========================================
  Files          98       98              
  Lines       19854    20114     +260     
==========================================
+ Hits        19269    19526     +257     
- Misses        585      588       +3