[Tune] Working with checkpoints user guide

[justinvyu]

This "working with checkpoints" guide has been converted to a Tune storage guide, and the content in the old guide was always more to do with syncing than teaching users how to create/load checkpoints.

The only information about creating trial checkpoints is here in the Tune API ref: https://docs.ray.io/en/master/tune/api/trainable.html

Once this guide is created, remove the URL redirect from tune-checkpoints -> tune-storage.

We should create a user guide called tune-checkpoints again for this common workflow. Here's what it could look like:

How to Save and Load Checkpoints in Ray Tune

Saving

For class + function trainables.

Use this content:

• https://docs.ray.io/en/master/tune/api/trainable.html#function-api-checkpointing
• https://docs.ray.io/en/master/tune/api/trainable.html#class-api-checkpointing

Loading

For class + function trainables.

Configurations

Explaining CheckpointConfig

• num_to_keep
• checkpoint frequency for class Trainable vs. an example of how to set it manually for function trainable

See Also

Checkpointing enables:

• Fault tolerance guide
• Storage options guide
• Batch prediction guide

Appendix: Experiment Checkpoints vs. Trial Checkpoints

Put this content in an appendix: https://docs.ray.io/en/releases-2.2.0/tune/tutorials/tune-checkpoints.html#two-different-types-of-tune-checkpoints