IVGCVSW-2458 Refactor the Optimize function (Network.cpp) so that
subgraphs are optimized by the backends

 * Added a new method OptimizeSubGraph to the backend interface
 * Refactored the Optimize function so that the backend-specific
   optimization is performed by the backend itself (through the new
   OptimizeSubGraph interface method)
 * Added a new ApplyBackendOptimizations function to apply the new
   changes
 * Added some new convenient constructors to the SubGraph class
 * Added AddLayer method and a pointer to the parent graph to the
   SubGraph class
 * Updated the sub-graph unit tests to match the changes
 * Added SelectSubGraphs and ReplaceSubGraphConnections overloads
   that work with sub-graphs
 * Removed unused code and minor refactoring where necessary

Change-Id: I46181794c6a9e3b10558944f804e06a8f693a6d0
diff --git a/src/backends/cl/ClBackend.cpp b/src/backends/cl/ClBackend.cpp
index 2f9dfa9..dfa5e7c 100644
--- a/src/backends/cl/ClBackend.cpp
+++ b/src/backends/cl/ClBackend.cpp
@@ -80,4 +80,13 @@
     return layerSupport;
 }
 
+IBackendInternal::SubGraphUniquePtr ClBackend::OptimizeSubGraph(const SubGraph& subGraph,
+                                                                bool& optimizationAttempted) const
+{
+    // Not trying to optimize the given sub-graph
+    optimizationAttempted = false;
+
+    return SubGraphUniquePtr{};
+}
+
 } // namespace armnn