allow different number of out-of-place args for isinplace

[ranocha]

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

This helps us at https://github.com/SKopecz/PositiveIntegrators.jl

[ChrisRackauckas]

What's this for?

[ranocha]

For PositiveIntegrators.jl, we need to partition the ODE right-hand side into (positive) production and (negative) destruction terms. Up to now, we have used two functions for this that can either be in-place (like production!(production_terms, u, p, t)) or out-of-place (like production_terms = production(u, p, t)). Thus, we could use isinplace as it is. To improve the performance of some applications, we want to use a single function with in-place form rhs!(production_terms, destruction_terms, u, p, t) and out-of-place form production_terms, destruction_terms = rhs(u, p, t). With the current version of isinplace, it's hardcoded that the out-of-place form has exactly one argument less than the in-place form. Thus, we cannot use it as it is right now for our new desired dispatches.

[ranocha]

The downgrade action as well as many integration tests fail already on master

[ranocha]

Is that something we can support with SciMLBase.jl, @ChrisRackauckas? The only other option we have is to replicate code in PositiveIntegrators.jl.

[ranocha]

Thanks!