Use broadcast for dropout

Should be fast enough on GPU now that it's not going to be an optimization target again for a while. Hopefully isn't meaningfully slower on CPU?

src/layers/normalise.jl

@@ -31,15 +31,14 @@ function Dropout(p)
   Dropout{typeof(p)}(p, true)
 end
 
+_dropout_kernel(y::T, p, q) where {T} = y > p ? T(1 / q) : T(0)
+
 function (a::Dropout)(x)
   a.active || return x
   y = similar(x)
   rand!(y)
-  q = 1 - a.p
-  @inbounds for i=1:length(y)
-    y[i] = y[i] > a.p ? 1 / q : 0
-  end
-  return y .* x
+  y .= _dropout_kernel.(y, a.p, 1 - a.p)
+  return x .* y
 end
 
 _testmode!(a::Dropout, test) = (a.active = !test)