Add lazy EMD solver with O(n) memory requirement

[nathanneike]

Add lazy EMD solver with on-the-fly distance computation

• Implement emd_c_lazy in C++ network simplex for memory-efficient OT
• Add lazy mode to emd2() accepting coordinates (X_a, X_b) instead of cost matrix
• Support sqeuclidean, euclidean, and cityblock metrics
• Add restrict for SIMD optimization
• Remove debug output from network_simplex_simple.h
• Add tests for lazy solver and metric variants

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and context / Related issue

Addresses memory limitations when computing OT with large point clouds. Instead of pre-computing and storing the full n×m cost matrix, the lazy solver computes distances on-the-fly during the network simplex algorithm. This reduces memory from O(nm) to O(n+m) while maintaining exact EMD solutions.

How has this been tested

• All existing tests pass (1031 passed, 69 skipped)
• Added new tests in test_solvers.py: test_solve_sample_lazy and test_solve_sample_lazy_emd
• Verified correctness against standard EMD solver
• Tested all three metrics (sqeuclidean, euclidean, cityblock)
• Confirmed SIMD optimization with compiler output analysis

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

❌ Patch coverage is 91.45299% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.97%. Comparing base (4c49769) to head (58c520c).
⚠️ Report is 1 commits behind head on master.

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HEAD has 19 uploads less than BASE

Flag	BASE (4c49769)	HEAD (58c520c)
	20	1

Additional details and impacted files

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##           master     #788       +/-   ##
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- Coverage   97.07%   84.97%   -12.10%     
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  Files         107      107               
  Lines       22156    22249       +93     
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- Hits        21507    18906     -2601     
- Misses        649     3343     +2694     

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[rflamary]

Nearly there, this is a wonderful PR that will allows solvers on very large data (at the cost of computing time)

[rflamary]

since it is sparse, it could be true by default

[nathanneike]

The solver here is only for dense ? So not sure why it should be true by defautl