[MRG] Partial Entropic FGW solvers

[cedricvincentcuaz]

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]

Codecov Report

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

Project coverage is 97.08%. Comparing base (200322b) to head (c3264c1).

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #702    +/-   ##
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  Coverage   97.08%   97.08%            
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  Files          98       98            
  Lines       19937    20105   +168     
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+ Hits        19355    19519   +164     
- Misses        582      586     +4