Latest version broke DI tests

[gdalle]

Hi @brianguenter,
Opening this to keep track until I can produce an MWE. The latest version of your package led to this CI failure: https://github.com/JuliaDiff/DifferentiationInterface.jl/actions/runs/12825528680/job/35763670206
It has to do with in-place operators and matrix variables (vectors work fine). Any idea where this could come from?

[gdalle]

The operators concerned are hvp and derivative. Here is an MWE:

julia> using DifferentiationInterface

julia> import FastDifferentiation

julia> f(x) = sum(abs2, x)
f (generic function with 1 method)

julia> backend = AutoFastDifferentiation()
AutoFastDifferentiation()

julia> x, dx, dg = ones(2, 3), ones(2, 3), zeros(2, 3);

julia> hvp!(f, (dg,), backend, x, (dx,))
ERROR: ArgumentError: The in place result array does not have the expected size. Expected size: (6,). Actual size: (2, 3).
Stacktrace:
 [1] macro expansion
   @ ~/.julia/packages/FastDifferentiation/7my9Z/src/CodeGeneration.jl:199 [inlined]
 [2] macro expansion
   @ ~/.julia/packages/RuntimeGeneratedFunctions/M9ZX8/src/RuntimeGeneratedFunctions.jl:163 [inlined]
 [3] macro expansion
   @ ./none:0 [inlined]
 [4] generated_callfunc
   @ ./none:0 [inlined]
 [5] (::RuntimeGeneratedFunctions.RuntimeGeneratedFunction{…})(::Matrix{…}, ::Vector{…})
   @ RuntimeGeneratedFunctions ~/.julia/packages/RuntimeGeneratedFunctions/M9ZX8/src/RuntimeGeneratedFunctions.jl:150
 [6] hvp!(f::Function, tg::Tuple{…}, prep::DifferentiationInterfaceFastDifferentiationExt.FastDifferentiationHVPPrep{…}, ::AutoFastDifferentiation, x::Matrix{…}, tx::Tuple{…})
   @ DifferentiationInterfaceFastDifferentiationExt ~/.julia/packages/DifferentiationInterface/7HVn3/ext/DifferentiationInterfaceFastDifferentiationExt/onearg.jl:441
 [7] hvp!(::typeof(f), ::Tuple{Matrix{Float64}}, ::AutoFastDifferentiation, ::Matrix{Float64}, ::Tuple{Matrix{Float64}})
   @ DifferentiationInterface ~/.julia/packages/DifferentiationInterface/7HVn3/src/fallbacks/no_prep.jl:139
 [8] top-level scope
   @ ~/Documents/GitHub/Julia/DifferentiationInterface.jl/DifferentiationInterface/test/playground.jl:7
Some type information was truncated. Use `show(err)` to see complete types.

(@di) pkg> st
Status `~/.julia/environments/di/Project.toml`
  [a0c0ee7d] DifferentiationInterface v0.6.29
  [eb9bf01b] FastDifferentiation v0.4.3