[WIP] Linear Circular OT

[clbonet]

Types of changes

This PR aims to add the Linear Circular OT distance (with uniform measure on the circle as reference measure) introduced in LCOT: Linear Circular Optimal Transport
and its sliced counterpart introduced in Linear Spherical Sliced Optimal Transport: A Fast Metric for Comparing Spherical Data
.

Changes:

• Added the ot.solver_1d.linear_circular_embedding and ot.solver_1d.linear_circular_ot functions to compute the Linear Circular OT distance.
• Added the ot.sliced.linear_sliced_wasserstein_sphere function to compute its sliced counterpart.
• Refactored the code for the spherical sliced distances, adding the functions ot.sliced.get_projections_sphere and ot.sliced.projection_sphere_to_circle.
• Added tests for these functions in test/test_1d_solver and test/test_sliced.
• Added the Linear Circular OT distance in the example plot_compute_wasserstein_circle.py
• Modified the default behavior of ot.wasserstein_circle to always use ot.solver_1d.binary_search_circle for any p, which seems to give better results even for p=1.
• Improved the visualization of the sphere in the example plot_ssw_unif_torch.py.

Motivation and context / Related issue

The Linear Circular OT distance with uniform measure on the circle is very fast to compute, as the OT maps can be computed in closed-forms. It is thus faster to compute than the true wasserstein distance on the circle.

How has this been tested (if it applies)

I added tests of these functions in test/test_1d_solver and test/test_sliced.

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 97.10%. Comparing base (61e9af4) to head (52871e5).

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #736    +/-   ##
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  Coverage   97.09%   97.10%            
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  Files         100      100            
  Lines       20432    20674   +242     
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+ Hits        19839    20075   +236     
- Misses        593      599     +6     

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