Make get_initial_values differentiate

[DhairyaLGandhi]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

nlsol has 2 incompatible graphs that assign to the same variable. This way we can use a simple indirection to disambiguate between them. For some reason this causes the Ref with the extra field access in initprob, so using initdata instead.

Add any other context about the problem here.