Explicitly unthunk in a few rules (#670)

* unthunk in rrule for +

* unthunk in unbroadcast

* v1.44.5

Author: mcabbott

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRules"
 uuid = "082447d4-558c-5d27-93f4-14fc19e9eca2"
-version = "1.44.4"
+version = "1.44.5"
 
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"

src/rulesets/Base/arraymath.jl

@@ -415,7 +415,8 @@ frule((_, ΔAs...), ::typeof(+), As::AbstractArray...) = +(As...), +(ΔAs...)
 function rrule(::typeof(+), arrs::AbstractArray...)
     y = +(arrs...)
     arr_axs = map(axes, arrs)
-    function add_pullback(dy)
+    function add_pullback(dy_raw)
+        dy = unthunk(dy_raw)  # reshape will otherwise unthunk N times
         return (NoTangent(), map(ax -> reshape(dy, ax), arr_axs)...)
     end
     return y, add_pullback

src/rulesets/Base/broadcast.jl

@@ -316,7 +316,8 @@ rrule(::typeof(broadcasted), ::typeof(complex), x::Number) = rrule(complex, x) |
 # When sizes disagree, broadcasting gradient uses `unbroadcast` to reduce to correct shape.
 # It's sometimes a little wasteful to allocate a too-large `dx`, but difficult to make more efficient.
 
-function unbroadcast(x::Base.AbstractArrayOrBroadcasted, dx)
+function unbroadcast(x::Base.AbstractArrayOrBroadcasted, dx_raw)
+    dx = unthunk(dx_raw)
     N = ndims(dx)
     if length(x) == length(dx)
         ProjectTo(x)(dx)  # handles trivial reshapes, offsets, structured matrices, row vectors
@@ -327,7 +328,8 @@ function unbroadcast(x::Base.AbstractArrayOrBroadcasted, dx)
 end
 unbroadcast(x::Base.AbstractArrayOrBroadcasted, dx::AbstractZero) = dx
 
-function unbroadcast(x::T, dx) where {T<:Tuple{Vararg{Any,N}}} where {N}
+function unbroadcast(x::T, dx_raw) where {T<:Tuple{Vararg{Any,N}}} where {N}
+    dx = unthunk(dx_raw)
     val = if N == length(dx)
         dx
     else