Add top-k mask and some minor fixes

What's Changed

• A few changes to increase Coverage by @marcocuturi in #390
• rename ent_reg_cost in sinkhorn solver by @marcocuturi in #391
• Increase coverage of NN modules by including init in tests by @marcocuturi in #392
• add tests for plots in potentials by @marcocuturi in #393
• test Plot class in tools module to increase coverage by @marcocuturi in #394
• Add soft topk_mask operator, fix ranks docs and tests by @marcocuturi in #396
• Add relative_epsilon option to GromovWasserstein by @michalk8 in #355
• [ENH]: Fix misc typos in soft_sort doc by @pierreablin in #397
• Update docs by @michalk8 in #398
• add ability to add titles on animation plots by @marcocuturi in #400
• Update contributing visibility by @michalk8 in #399
• update nb by @marcocuturi in #403
• Fix not being able to jit the soft quantile by @michalk8 in #408
• Test default_progress_fn, skip types.py by @michalk8 in #404

Full Changelog: 0.4.2...0.4.3