Feature Request: Save N epoch and N steps, comma seperated

We need to be able to save only certain epochs and steps

Like save epoch 30,35,40,45 and no others
or
Save step 300,400,500 and no others

Can you please add this option? Thank you @kohya-ss

This became super important for FLUX training since each checkpoint is 24 GB

This is for saving checkpoints but saving state option this way would be nice as well

The issue names no files, tests, or entry points. Locate the checkpoint-saving and training-state-saving paths first, then trace how epoch and step intervals are handled; done means supporting comma-separated epoch or step selections without saving other requested checkpoints, with coverage for both checkpoint and state-saving behavior.

Written by the indexing model from the issue text.