[WIP] Slicing UOT

[clbonet]

Types of changes

This PR aims at implementing the solver for 1D UOT with Frank-Wolfe (Faster Unbalanced Optimal Transport: Translation invariant Sinkhorn and 1-D Frank-Wolfe), as well as the Sliced Unbalanced OT and Unbalanced sliced OT distances (Slicing Unbalanced Optimal Transport).

Changes:

• Added the ot.solver_1d.emd_1d_dual function to compute the dual potentials in 1D efficiently (still some bugs)
• Added the ot.solver_1d.emd_1d_dual_backprop function, which computes the dual potentials in 1D by using backpropagation over the 1D wasserstein distance (in pytorch and jax).
• Added ot.unbalanced._solver_1d.uot_1d to compute the 1D UOT problems with Frank-Wolfe
• Added ot.unbalanced._sliced.sliced_unbalanced_ot to compute SUOT
• Added ot.unbalanced._sliced.unbalanced_sliced_ot to compute USOT

Motivation and context / Related issue

These functions allow to compute unbalanced OT problems efficiently.

How has this been tested (if it applies)

It still needs to be properly tested.

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.