Refactor ChainRulesCoreExt into separate files

ext/TensorKitChainRulesCoreExt/constructors.jl

@@ -17,8 +17,10 @@
     return copy(t), copy_pullback
 end
 
+# this rule does not work for generic symmetries, as we currently have no way to
+# project back onto the symmetric subspace
 function ChainRulesCore.rrule(::typeof(Base.convert), T::Type{<:Array},
-                              t::AbstractTensorMap)
+                              t::TrivialTensorMap)
     A = convert(T, t)
     function convert_pullback(ΔA)
         ∂t = TensorMap(unthunk(ΔA), codomain(t), domain(t))
@@ -27,21 +29,21 @@
     return A, convert_pullback
 end
 
 function ChainRulesCore.rrule(::typeof(Base.convert), ::Type{Dict}, t::AbstractTensorMap)
     out = convert(Dict, t)
     function convert_pullback(c)
         if haskey(c, :data) # :data is the only thing for which this dual makes sense
             dual = copy(out)
             dual[:data] = c[:data]
             return (NoTangent(), NoTangent(), convert(TensorMap, dual))
         else
             # instead of zero(t) you can also return ZeroTangent(), which is type unstable
             return (NoTangent(), NoTangent(), zero(t))
         end
     end
     return out, convert_pullback
 end
 function ChainRulesCore.rrule(::typeof(Base.convert), ::Type{TensorMap},
                               t::Dict{Symbol,Any})
     return convert(TensorMap, t), v -> (NoTangent(), NoTangent(), convert(Dict, v))
 end

test/ad.jl

@@ -143,8 +143,10 @@ Vlist = ((ℂ^2, (ℂ^3)', ℂ^3, ℂ^2, (ℂ^2)'),
         test_rrule(copy, T1)
         test_rrule(copy, T2)
 
-        test_rrule(convert, Array, T1)
-        test_rrule(TensorMap, convert(Array, T1), space(T1))
+        T1 isa TrivialTensorMap && test_rrule(convert, Array, T1)
+        T2 isa TrivialTensorMap && test_rrule(TensorMap, convert(Array, T1), space(T1))
+        # TODO: can we make these methods/tests work for generic symmetries?
+        # the main problem here is finitedifferencing generates non-symmetric entries
     end
 
     @timedtestset "Basic Linear Algebra with scalartype $T" for T in (Float64, ComplexF64)