[FEAT] Alpha differentiability in semirelaxed_gromov_wasserstein2

[SoniaMaz8]

Types of changes

Addition of differentiability over the trade-off parameter $\alpha$ in the semirelaxed_gromov_wasserstein2 function in ot/gromov/_semirelaxed.py.

Motivation and context / Related issue

How has this been tested (if it applies)

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.

[rflamary]

Hello @SoniaMaz8 that is great.

You should also check that the gradients are properly set during a .backward() in pytorh by editing the following test:

POT/test/test_gromov.py

Line 1714 in 5faa4fb

def test_semirelaxed_fgw2_gradients():