Types of changes

In this PR we propose the following contribution

• generalization of Unbalaced BFGS solver to TV marginal penalization and general regularizer terms (given as a tuple of value, grad functions)
• Generalization of ot.solve to handle TV marginal penalization for unbalanced solver and general regularization for unbalanced and exact ot solver (using cg)
• Example of a grid of regularization and marginal penalization in the doc

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.