fix: define Zygote.@adjoint to fall back to ChainRulesCore.rrule

Author: AayushSabharwal

Project.toml

@@ -50,7 +50,7 @@ SciMLBasePartialFunctionsExt = "PartialFunctions"
 SciMLBasePyCallExt = "PyCall"
 SciMLBasePythonCallExt = "PythonCall"
 SciMLBaseRCallExt = "RCall"
-SciMLBaseZygoteExt = "Zygote"
+SciMLBaseZygoteExt = ["Zygote", "ChainRulesCore"]
 
 [compat]
 ADTypes = "0.2.5,1.0.0"

ext/SciMLBaseZygoteExt.jl

@@ -3,6 +3,7 @@ module SciMLBaseZygoteExt
 using Zygote
 using Zygote: @adjoint, pullback
 import Zygote: literal_getproperty
+import ChainRulesCore
 using SciMLBase
 using SciMLBase: ODESolution, remake,
                  getobserved, build_solution, EnsembleSolution,
@@ -40,6 +41,12 @@ import SciMLStructures
     VA[i, j], ODESolution_getindex_pullback
 end
 
+struct ZygoteConfig <: ChainRulesCore.RuleConfig{ChainRulesCore.HasReverseMode} end
+
+@adjoint function Base.getindex(VA::ODESolution, sym, j::Integer)
+    Zygote.ChainRulesCore.rrule(ZygoteConfig(), getindex, VA, sym, j)
+end
+
 @adjoint function EnsembleSolution(sim, time, converged, stats)
     out = EnsembleSolution(sim, time, converged, stats)
     function EnsembleSolution_adjoint(p̄::AbstractArray{T, N}) where {T, N}