Avoid std::deque<Input> before Input type is complete (#183)

src/ngraph/descriptor/output.hpp

@@ -17,11 +17,18 @@
 #include <memory>
 #include <set>
 
+#include "ngraph/descriptor/input.hpp"
 #include "ngraph/descriptor/tensor_view.hpp"
-#include "ngraph/node.hpp"
 
 namespace ngraph
 {
+    // The forward declaration of Node is needed here because Node has a deque of
+    // Outputs, and Output is an incomplete type at this point. STL containers of
+    // incomplete type have undefined behavior according to the C++11 standard, and
+    // in practice including node.hpp here was causing compilation errors on some
+    // systems (namely macOS).
+    class Node;
+
     namespace descriptor
     {
         // Describes an output tensor of an op