Merge #907

907: Change `gate` function to `view` instead of copy r=MikeInnes a=janEbert

This speeds up code with large inputs by quite a lot. I only added it to the function accepting an `AbstractVector` as input as copying matrices may be faster than viewing them due to caching (they are sliced per row so will the data will not necessarily have a low stride).

Co-authored-by: janEbert <janpublicebert@posteo.net>

src/layers/recurrent.jl

@@ -1,5 +1,5 @@
 gate(h, n) = (1:h) .+ h*(n-1)
-gate(x::AbstractVector, h, n) = x[gate(h,n)]
+gate(x::AbstractVector, h, n) = @view x[gate(h,n)]
 gate(x::AbstractMatrix, h, n) = x[gate(h,n),:]
 
 # Stateful recurrence