[Fix] Gradient scaling in Partial GW solver

[yikun-baio]

Types of changes

I modify the code ot.partial.partial_gromov_wasserstein

Motivation and context / Related issue

There seems to be an inconsistency between ot.partial.partial_gromov_wasserstein and the line search section in the paper [29]. I fixed this section. In addition, I have made minor change to the initial guess in the partial-GW solver since the original initial guess is np.out(p,q), which might not be suitable for unbalanced case, i.e. |p|\neq |q|.

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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.