[WIP] Sparse emd implementation

[nathanneike]

Types of changes

• New feature (non-breaking change which adds functionality)
• Tests (new tests or changes to existing tests)

Motivation and context / Related issue

This PR implements a sparse EMD solver for memory-efficient optimal transport when the cost matrix has many infinite or forbidden edges (e.g., k-NN graphs, sparse networks).

Problem: The current dense EMD solver requires O(n²) memory for the full cost matrix, which becomes prohibitive for large-scale
problems even when most edges are forbidden.

Solution: This PR adds a sparse bipartite graph solver that only stores edges with finite costs, reducing memory usage from O(n²) to O(E) where E is the number of edges.

Use cases:

• k-NN graph optimal transport
• Large-scale sparse matching problems
• Network flow with forbidden edges

How has this been tested

Unit Tests

Added two comprehensive tests in test/test_ot.py:

• test_emd_sparse_vs_dense() - Verifies sparse and dense solvers produce identical transport matrices
• test_emd2_sparse_vs_dense() - Verifies sparse and dense solvers produce identical costs

Both tests use the augmented k-NN approach:

1. Create initial k-NN sparse graph
2. Solve with dense solver to identify needed edges
3. Augment graph with those edges
4. Compare both solvers on identical graph structure

Test results: All 50 tests in test/test_ot.py pass

Verification

• Costs match between solvers
• Marginal constraints satisfied for both solvers
• No regression in existing tests

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts). TODO: Add documentation
• 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. TODO: Will add once ready for merge

TODO before [MRG]:

• Add example script in examples/ folder demonstrating sparse solver usage
• Add documentation explaining when to use sparse vs dense
• Performance benchmarks comparing memory usage and runtime
• Update RELEASES.md

Feedback requested:

• Is the API design appropriate? (using sparse=True parameter)
• Should we add more comprehensive tests?
• Any concerns about the C++ implementation approach?