[tune](deps): Bump ax-platform in /python/requirements (#14398)
[tune](deps): Bump ax-platform in /python/requirements (#14398)

Bumps [ax-platform](https://github.com/facebook/Ax) from 0.1.19 to 0.1.20.
- [Release notes](https://github.com/facebook/Ax/releases)
- [Changelog](https://github.com/facebook/Ax/blob/master/CHANGELOG.md)
- [Commits](https://github.com/facebook/Ax/compare/0.1.19...v0.1.20)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
ray-project/ray,pcmoritz/ray-1,ray-project/ray,pcmoritz/ray-1,ray-project/ray,ray-project/ray,ray-project/ray,pcmoritz/ray-1,pcmoritz/ray-1,ray-project/ray,pcmoritz/ray-1,ray-project/ray,pcmoritz/ray-1,pcmoritz/ray-1,ray-project/ray,pcmoritz/ray-1