fix: remove linear indexing from getindex adjoints

[AayushSabharwal]

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

Add any other context about the problem here.

[vchuravy]

function Adapt.adapt_structure(to, VA::AbstractVectorOfArray)
    Adapt.adapt(to, Array(VA))
end

This definition was surprising to me. I epected it to return a VectorOfArray whose underlying tuple is adapted.

[ChrisRackauckas]

It doesn't have an underlying tuple, it's an array of arrays. But yes, that is probably not right, it probably should recurse running adapt.

[ChrisRackauckas]

do you have a test case that is sensitive to this?

[ChrisRackauckas]

I made #434, but the tricky thing is always that GPUArrays cannot have other GPUArrays, so the structure would need to be Array of GPUArrays and I think VectorOfArray works out in that case?