propagate sinkdiv to Monge gap implementation

[marcocuturi]

make sure sinkhorn_divergence.sinkdiv is used in Monge gap related scripts / tests / tutorials

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.90%. Comparing base (86e8f2c) to head (9455013).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #597      +/-   ##
==========================================
- Coverage   88.17%   87.90%   -0.28%     
==========================================
  Files          73       73              
  Lines        7770     7770              
  Branches      556      556              
==========================================
- Hits         6851     6830      -21     
- Misses        778      799      +21     
  Partials      141      141              

Files with missing lines	Coverage Δ
src/ott/neural/methods/monge_gap.py	91.20% <ø> (ø)

... and 1 file with indirect coverage changes

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