graph support

src/backend/tensorflow/graph.jl

@@ -12,6 +12,7 @@ graph(::typeof(+), args...) = +(args...)
 graph(::typeof(softmax), x) = nn.softmax(x)
 graph(::typeof(relu), x) = nn.relu(x)
 graph(::typeof(tanh), x) = tanh(x)
+graph(::typeof(σ), x) = nn.sigmoid(x)
 
 # reshape hack due to https://github.com/malmaud/TensorFlow.jl/issues/79
 batchsize(x::Tensor) = reduce_sum(slice(TensorFlow.shape(x), [0], [1]))
@@ -26,6 +27,8 @@ graph(c::Conv2D, x) =
 graph(p::MaxPool, x) =
   nn.max_pool(x, [1, p.size..., 1], [1, p.stride..., 1], "VALID")
 
+graph(::Flow.Group, xs...) = (xs...,)
+
 type Op
   f
   shape