[MRG] Stereographic Spherical Sliced Wasserstein

[huytransformer]

Types of changes

This PR aims to add the Stereographic Spherical Sliced Wasserstein distance and its rotationally invariant extension introduced in Stereographic Spherical Sliced Wasserstein Distances.

Changes:

• Added the ot.sliced.stereographic_sliced_wasserstein_sphere function to compute the S3W distance, and the RI-S3W distance with the n_rotations argument. The amortized version ARI-S3W can be computed by pregenerating a pool of rotations with ot.sliced.get_random_rotations and passing a random subset with the rotations argument.
• Added the ot.sliced.get_random_rotations function to sample rotations uniformly on SO(d), used for RI-S3W.
• Added tests for these functions in test/sliced/test_spherical_sliced.py.

Motivation and context / Related issue

The S3W distances are fast alternatives to ot.sliced_wasserstein_sphere for comparing distributions on the sphere: after a stereographic projection composed with a near-isometric map, they only require computing sliced Wasserstein distances in R^{d-1}, and they support all backends including tensorflow.

How has this been tested (if it applies)

I added tests of these functions in test/sliced/test_spherical_sliced.py.

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.