Allow subsetting Initials in subset_tunables

[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

cc @SebastianM-C can you check if this is sufficient for your use cases, and we can add a test then

Add any other context about the problem here.

[ChrisRackauckas]

We should make sure this gets appropriately tested in SciMLSensitivity too.

[SebastianM-C]

yeah, it seems to work for me (on a simple example at least)

[SebastianM-C]

Btw, I noticed that subset_tunables does not remove Initials. Does this affect the performance or it's not an issue?

[ChrisRackauckas]

That will not effect the performance of adjoints much, but it will forward mode

[AayushSabharwal]

Initials are not tunables. Why exactly do we need this?

[SebastianM-C]

I'm not sure if we need this or not, I just noticed that Initials show up in tunable_parameters(sys).

[AayushSabharwal]

Oh, that's a lie. I'm pretty sure when I added initials and the initial_parameters = true keyword, I just did a find-replace of all parameters(sys) to parameters(sys; initial_parameters = true). This is probably a remnant of that.

[DhairyaLGandhi]

Does that mean this can be closed since it won't be actionable