add Dotp cost and corresponding test

[marcocuturi]

corresponds to 0.5 * SqEuclidean cost without norms.

Simple test in Sinkhorn comparing to Hungarian output was missing, tested the cost there.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.17%. Comparing base (9ae02ea) to head (9821844).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ott/geometry/costs.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #615      +/-   ##
==========================================
- Coverage   87.17%   87.17%   -0.01%     
==========================================
  Files          75       75              
  Lines        7761     7767       +6     
  Branches      538      538              
==========================================
+ Hits         6766     6771       +5     
- Misses        851      852       +1     
  Partials      144      144              

Files with missing lines	Coverage Δ
src/ott/geometry/costs.py	97.05% <83.33%> (-0.23%)	⬇️

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[michalk8]

LGTM, thank you!