Revert "remove flatten"

This reverts commit d91c203.

perf/vgg.jl

@@ -38,7 +38,7 @@ function vgg16()
         Conv((3, 3), 512 => 512, relu, pad=(1, 1), stride=(1, 1)),
         BatchNorm(512),
         MaxPool((2,2)),
-        Flux.flatten,
+        flatten,
         Dense(512, 4096, relu),
         Dropout(0.5),
         Dense(4096, 4096, relu),

src/layers/stateless.jl

@@ -1,3 +1,31 @@
+"""
+    flatten(x::AbstractArray)
+
+Reshape arbitrarly-shaped input into a matrix-shaped output,
+preserving the size of the last dimension.
+
+See also [`unsqueeze`](@ref).
+
+# Examples
+```jldoctest
+julia> rand(3,4,5) |> Flux.flatten |> size
+(12, 5)
+
+julia> xs = rand(Float32, 10,10,3,7);
+
+julia> m = Chain(Conv((3,3), 3 => 4, pad=1), Flux.flatten, Dense(400 => 33));
+
+julia> xs |> m[1] |> size
+(10, 10, 4, 7)
+
+julia> xs |> m |> size
+(33, 7)
+```
+"""
+function flatten(x::AbstractArray)
+  return reshape(x, :, size(x)[end])
+end
+
 """
     normalise(x; dims=ndims(x), ϵ=1e-5)