Types of changes

• Implement Entropic partial FGW solvers using projected gradient descent in ot.gromov.entropic_partial_fused_gromov_wasserstein and ot.gromov.entropic_partial_fused_gromov_wasserstein2.
• Add these solvers to ot.solve_gromov. Add a warning when unbalanced=None and unbalanced_type="partial".
• New exemples with (entropic) Partial GW and FGW in examples/gromov/plot_partial_fgw.py

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 Report

Attention: Patch coverage is 98.86364% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.81%. Comparing base (c128104) to head (e2c811b).
Report is 1 commits behind head on master.

@@            Coverage Diff             @@
##           master     #702      +/-   ##
==========================================
- Coverage   97.08%   96.81%   -0.27%     
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  Files         100      100              
  Lines       19947    20115     +168     
==========================================
+ Hits        19365    19474     +109     
- Misses        582      641      +59