make activations zygote friendly

src/layers/basic.jl

@@ -44,17 +44,15 @@ end
 # it might be replaced in the future for better performance
 # see issue https://github.com/FluxML/Flux.jl/issues/702
 # Johnny Chen -- @johnnychen94
+# only slightly changed to better handle interaction with Zygote @dsweber2
 """
     activations(c::Chain, input)
 Calculate the forward results of each layers in Chain `c` with `input` as model input.
 """
 function activations(c::Chain, input)
-  rst = []
-  for l in c
-    x = get(rst, length(rst), input)
-    push!(rst, l(x))
-  end
-  return rst
+  buffed = accumulate!((x,y)->y(x), Zygote.Buffer([], length(c)),
+                       [l for l in c], dims=1, init=input) 
+  return copy(buffed)
 end