Don't allow sum of anything in rrule

Author: oxinabox

src/rulesets/Base/mapreduce.jl

@@ -57,7 +57,7 @@ function frule(::typeof(sum), x)
     return sum(x), sum_pushforward
 end
 
-function rrule(::typeof(sum), x)
+function rrule(::typeof(sum), x::AbstractArray{<:Real})
     function sum_pullback(ȳ)
         return (NO_FIELDS, @thunk(fill(ȳ, size(x))))
     end
@@ -67,15 +67,15 @@ end
 function rrule(::typeof(sum), f, x::AbstractArray{<:Real}; dims=:)
     y, mr_pullback = rrule(mapreduce, f, Base.add_sum, x; dims=dims)
     function sum_pullback(ȳ)
-        NO_FIELDS, DNE(), last(mr_pullback(ȳ))
+        return NO_FIELDS, DNE(), last(mr_pullback(ȳ))
     end
     return y, sum_pullback
 end
 
 function rrule(::typeof(sum), x::AbstractArray{<:Real}; dims=:)
     y,  inner_pullback = rrule(sum, identity, x; dims=dims)
     function sum_pullback(ȳ)
-        NO_FIELDS, last(inner_pullback(ȳ))
+        return NO_FIELDS, last(inner_pullback(ȳ))
     end
     return y, sum_pullback
 end