[FIX] index fail during save in python

[deepakdhull80]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
  • Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones?

[daxpryce]

@deepakdhull80 I don't think this is quite the right way to fix this. I just made some changes in #404 that should address the failure to save that I think are the correct way to do this for now, without impacting our Cxx-first users (not everyone is using this via diskannpy yet, and adding it in here may break the CLI utilities or libs already built).

Regardless, thank you so much for submitting a PR for this; we really appreciate your contribution!

I hope to have rc5 released today or early tomorrow which should include a working save on dynamic index.