Cannot compute nested gradients with reverse mode AD

[bolgarbe]

The following fails in Diffractor (works in Zygote but only in mixed mode):

julia> using Diffractor: var"'"

julia> A = randn(3,3);

julia> f(x) = sum(A*x);

julia> g(x) = sum(x .* f'(x));

julia> g'(ones(3))
ERROR: MethodError: no method matching _adjoint_vec_pullback(::ChainRulesCore.ZeroTangent)
Closest candidates are:
  _adjoint_vec_pullback(::ChainRulesCore.Tangent) at /home/bj0rn/.julia/packages/ChainRules/xaVoS/src/rulesets/LinearAlgebra/structured.jl:119
  _adjoint_vec_pullback(::AbstractMatrix) at /home/bj0rn/.julia/packages/ChainRules/xaVoS/src/rulesets/LinearAlgebra/structured.jl:120
  _adjoint_vec_pullback(::ChainRulesCore.AbstractThunk) at /home/bj0rn/.julia/packages/ChainRules/xaVoS/src/rulesets/LinearAlgebra/structured.jl:121
Stacktrace:
 [1] ∂⃖¹₁times_pullback
   @ ./none:1
 [2] (::Diffractor.∂⃖rruleB{1, 1})(::ChainRulesCore.ZeroTangent, ::Vararg{Any})
   @ Diffractor ~/.julia/packages/Diffractor/EwwgG/src/stage1/generated.jl:64
 [3] ∂⃖²₂f
   @ ./none:1
 [4] (::Diffractor.∂⃖weaveInnerOdd{1, 1})(Δ::Tuple{ChainRulesCore.ZeroTangent, Vector{Float64}})
   @ Diffractor ~/.julia/packages/Diffractor/EwwgG/src/stage1/generated.jl:64
 [5] ∂⃖¹₁PrimeDerivativeBack
   @ ./none:1
 [6] ∂⃖¹₁g
   @ ./none:1
 [7] (::Diffractor.PrimeDerivativeBack{1, typeof(g)})(x::Vector{Float64})
   @ Diffractor ~/.julia/packages/Diffractor/EwwgG/src/interface.jl:160
 [8] top-level scope
   @ REPL[5]:1

[mzgubic]

I think this is a special case of #25

[Keno]

The error as reported as fixed, but it then gets lost in higher-order AD of broadcast which doesn't quite work yet. However, the use of broadcast in the sum rrule is somewhat gratuitous anyway, so this should work after JuliaDiff/ChainRules.jl#493.

[Keno]

Fixed by JuliaDiff/ChainRules.jl#494