chore: don't return structural tangent

ext/SciMLBaseZygoteExt.jl

@@ -6,9 +6,10 @@ import Zygote: literal_getproperty
 using SciMLBase
 using SciMLBase: ODESolution, remake,
  getobserved, build_solution, EnsembleSolution,
- NonlinearSolution, AbstractTimeseriesSolution
+ NonlinearSolution, AbstractTimeseriesSolution,
+ SciMLStructures
 using SymbolicIndexingInterface: symbolic_type, NotSymbolic, variable_index, is_observed,
- observed, parameter_values
+ observed, parameter_values, state_values, current_time
 using RecursiveArrayTools
 
 # This method resolves the ambiguity with the pullback defined in
@@ -111,10 +112,13 @@ end
  function ODESolution_getindex_pullback(Δ)
  i = symbolic_type(sym) != NotSymbolic() ? variable_index(VA, sym) : sym
  if is_observed(VA, sym)
- y, back = Zygote.pullback(VA) do sol
- f = observed(sol, sym)
- p = parameter_values(sol)
- f.(sol.u, Ref(p), sol.t)
+ f = observed(VA, sym)
+ p = parameter_values(VA)
+ tunables, _, _ = SciMLStructures.canonicalize(SciMLStructures.Tunable(), p)
+ u = state_values(VA)
+ t = current_time(VA)
+ y, back = Zygote.pullback(u, tunables) do u, tunables
+ f.(u, Ref(tunables), t)
  end
  gs = back(Δ)
  (gs[1], nothing)
@@ -154,8 +158,7 @@ function not_obs_grads(VA::ODESolution{T}, sym, not_obss_idx, i, Δ) where {T}
  end
  end
 
- nt = Zygote.nt_nothing(VA)
- Zygote.accum(nt, (u = Δ′,))
+ Δ′
 end
 
 @adjoint function Base.getindex(
@@ -171,6 +174,7 @@ end
  gs_not_obs = not_obs_grads(VA, sym, not_obs_idx, i, Δ)
 
  a = Zygote.accum(gs_obs[1], gs_not_obs)
+
  (a, nothing)
  end
  VA[sym], ODESolution_getindex_pullback

test/downstream/observables_autodiff.jl

@@ -35,15 +35,15 @@ sol = solve(prob, Tsit5())
  end
  du_ = [0.0, 1.0, 1.0, 1.0]
  du = [du_ for _ in sol.u]
- @test du == gs.u
+ @test du == gs
 
  # Observable in a vector
  gs, = gradient(sol) do sol
  sum(sum.(sol[[sys.w, sys.x]]))
  end
  du_ = [0.0, 1.0, 1.0, 2.0]
  du = [du_ for _ in sol.u]
- @test du == gs.u
+ @test du == gs
 end
 
 # DAE
@@ -84,7 +84,7 @@ end
  end
  du_ = [0.2, 1.0]
  du = [du_ for _ in sol.u]
- @test gs.u == du
+ @test gs == du
 end
 
 # @testset "Adjoints with DAE" begin