[MAINTAIN] orbax-checkpoint API does not expose user to Preservation Policy

[shehper]

In CheckpointMangerOptions docstring, it is said that max_to_keep, keep_time_interval, etc are deprecated and users should use preservation_policy instead. See here

However, orbax/checkpoint/orbax/checkpoint/__init__.py file does not expose users to PreservationPolicy class.

This is probably an oversight. Hence, I am opening this issue.

[orbax-dev]

The intention was for users to use the symbols from https://github.com/google/orbax/blob/main/checkpoint/orbax/checkpoint/checkpoint_managers.py, since there are rather a lot of different options that we wanted to avoid exposing directly on the main init file, which is already fairly cluttered.

[shoyer]

It would be nice to document these somewhere in the API docs

[orbax-dev]

True, I will make sure this is made clear.

[shehper]

Related to @shoyer 's comment, API Overview doc definesoptions with save_interval_steps and max_to_keep. It should probably be defined with PreservationPolicy instead.

If you are okay with it, I will be happy to submit a PR for this change. Thank you.

[cpgaffney1]

Right, some of this has gotten a bit outdated. Thank you, I appreciate the help!! Note that we don't have external contribution machinery properly set up at the moment, so we would have to manually copy your PR into an internal change in order to merge it.