Fix datasaver under future annotations

[ArnavBalyan]

Changes

• datasaver rejected dict returning functions when annotations are strings.
• Switched to typing.get_type_hints (same as dataloader)
• Now also accepts dict[str, T] and Dict[str, T]

How I tested this

• UT

Notes

• NA

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[ArnavBalyan]

cc @elijahbenizzy thanks!