[MRG] Fix bug SSW backend

[clbonet]

Types of changes

• Use nx.einsum in ot.sliced_wasserstein_sphere and ot.sliced_wasserstein_sphere_unif to project points on the circle
• Added the possibility to give projections as input of ot.sliced_wasserstein_sphere
• Added the method matmul in the backend, which use the same API as numpy
• Added test in test_backend for matmul and in test_sliced to check that ot.sliced_wasserstein_sphere returns the same value when used with the same projections on different backend

Motivation and context / Related issue

The function ot.sliced_wasserstein_sphere did not return the same values given the backend used.

How has this been tested (if it applies)

• Tests for ot.sliced_wasserstein_sphere have been added to check that it returns the same value when used with different backend but with the same projections
• Tests for nx.matmul have been added in test_backend to multiply batchs of matrices of the form (1,n,d,d) and (m,1,d,d)

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.