Added the SkipConnection layer and constructor

Added missing export

Corrected channel placement

Dimension 4 cannot be assumed to always be the Channel dimension

Deprecation of `treelike`

Code now makes use of `@treelike` macro instead of the deprecated `treelike` function (it worked on my end because I'm on Julia 0.7, while Julia 1.0 deprecated stuff)

Update basic.jl

Renaming to SkipConnection

* Update Flux.jl

* Update basic.jl

Updated `SkipConnection` with a `connection` field

I'm pretty sure I broke something now, but this PR should follow along these lines `cat` needs special treatment (the user can declare his own `concatenate` connection, but I foresee it's going to be used often so we can simply define special treatment)

Forgot to remove some rebasing text

Forgot to remove some more rebasing text

Removed local copy and default cat method from the function calls

Adjusted some more types for inference, could improve on this as well

Re-placed some left-over spaces

src/Flux.jl

@@ -6,7 +6,7 @@ using Base: tail
 using MacroTools, Juno, Requires, Reexport, Statistics, Random
 using MacroTools: @forward
 
-export Chain, Dense, Maxout, RNN, LSTM, GRU, Conv, ConvTranspose, MaxPool, MeanPool,
+export Chain, Dense, Maxout, RNN, LSTM, GRU, Conv, ConvTranspose, SkipConnection, MaxPool, MeanPool,
        DepthwiseConv, Dropout, AlphaDropout, LayerNorm, BatchNorm, InstanceNorm, GroupNorm, 
        params, mapleaves, cpu, gpu, f32, f64
 

src/layers/basic.jl

@@ -189,3 +189,33 @@ end
 function (mo::Maxout)(input::AbstractArray)
     mapreduce(f -> f(input), (acc, out) -> max.(acc, out), mo.over)
 end
+
+
+"""
+    SkipConnection(layers...)
+
+
+Creates a Skip Connection, which constitutes of a layer or Chain of consecutive layers
+and a shortcut connection linking the input to the block to the
+output through a user-supplied function.
+
+`SkipConnection` requires the output dimension to be the same as the input.
+"""
+
+struct SkipConnection
+  layers
+  connection::Function  #user can pass arbitrary connections here, such as `.+`
+end
+
+@treelike SkipConnection
+
+function (skip::SkipConnection)(input::AbstractArray)
+  #We apply the layers to the input and return the result of the application of the layers and the original input
+  skip.connection(skip.layers(input), input)
+end
+
+function Base.show(io::IO, b::SkipConnection)
+  print(io, "SkipConnection(")
+  join(io, b.layers, ", ")
+  print(io, ")")
+end