Fix problems related to unthunk_tangent for IdDict

Co-authored-by: Brian Chen <[email protected]>

src/compiler/chainrules.jl

@@ -1,6 +1,10 @@
 @inline unthunk_tangent(x::AbstractThunk) = wrap_chainrules_output(unthunk(x))
 @inline unthunk_tangent(x::AbstractArray{<:AbstractThunk}) = map(unthunk_tangent, x)
 unthunk_tangent(d::IdDict) = IdDict([unthunk_tangent(k) => unthunk_tangent(v) for (k, v) in d])
+function ChainRulesCore.rrule(::typeof(unthunk_tangent), d::IdDict)
+  unthunk_iddict_pullback(_) = (NoTangent(), ChainRulesCore.@not_implemented "unthunking IdDict")
+  return d, unthunk_iddict_pullback
+end
 @non_differentiable unthunk_tangent(::IdDict)