frule_via_ad tries to call NoTangent

[gdalle]

julia> using ChainRulesCore, Diffractor

julia> ChainRulesCore.frule_via_ad(Diffractor.DiffractorRuleConfig(), (NoTangent(), 1), exp, 1)
ERROR: MethodError: objects of type NoTangent are not callable
Stacktrace:
 [1] ∂☆nomethd(args::Any)
   @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/stage1/recurse_fwd.jl:66
 [2] macro expansion
   @ ~/.julia/packages/Diffractor/v1WuP/src/stage1/recurse_fwd.jl:0 [inlined]
 [3] (::Diffractor.∂☆recurse{…})(::Diffractor.TangentBundle{…}, ::Diffractor.TangentBundle{…})
   @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/stage1/recurse_fwd.jl:103
 [4] (::Diffractor.∂☆internal{…})(::Diffractor.TangentBundle{…}, ::Vararg{…})
   @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/stage1/forward.jl:114
 [5] frule_via_ad(::Diffractor.DiffractorRuleConfig, ::Tuple{NoTangent, Int64}, ::Function, ::Vararg{Any})
   @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/stage1/forward.jl:130
 [6] top-level scope
   @ ~/Work/GitHub/Julia/DifferentiationInterface.jl/test/diffractor.jl:18
Some type information was truncated. Use `show(err)` to see complete types.

[gdalle]

@adrhill this is why Diffractor tests fail