add gradients in place

src/layers/params.jl

@@ -9,12 +9,12 @@ state(p::Param) = p
 state(x) = x
 
 function accumulate!(p::Param, Δ)
-  p.Δx += Δ
+  p.Δx .+= Δ
   return p
 end
 
 function update!(p::Param, η)
-  p.x += p.Δx * η
+  p.x .+= p.Δx * η
   return p
 end