[WIP] Fix quantized (F)GW solvers

[cedricvincentcuaz]

Types of changes

Fix quantized (F)GW solvers that output mis-ordered indices in final OT plan (Issue #786 ).
Implies modifications of API, providing:

• new parameters part1 and part2 as inputs to quantized_fused_gromov_wasserstein_partitioned. These variables were computed via get_graph_partition but formatted as a list or array-like containing the node assignments. This has been reformatted as a list of array-like, where each array contains the indices of the nodes in the partition, to avoid redundant steps in the previous implementations.

• The construction of the transport plan in quantized_fused_gromov_wasserstein_partitioned is support by two hidden functions _build_full_transport_by_assignment (numpy, torch - mutable) and _build_full_transport_by_concatenation (other non-mutable backends) to leverage a more memory-efficient approach for mutable backends.

Motivation and context / Related issue

How has this been tested (if it applies)

• This ordering mis-matched was missed by previous tests because marginals were systematically instantiatied as uniform ones. I alternated between random marginals and uniform ones to be able to catch potential issues.
• Additional tests were integrated to

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.85%. Comparing base (3073aa0) to head (213c509).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #857   +/-   ##
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  Coverage   96.85%   96.85%           
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  Files         128      128           
  Lines       26161    26161           
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  Hits        25337    25337           
  Misses        824      824           

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