IVGCVSW-3132 Reintroduce SubGraph definition, but deprecated

 * Restored old SubGraph class definition as an alias of SubgraphView
   for backward compatibility
 * Restored SubGraphUniquePtr
 * Restored CreateSubGraphConverter method (and the corresponding
   ISubGraphConverterPtr type) as a deprecated method that's been
   removed from the backend interface
 * Chaged the defautl implementation of OptimizeSubgraphView to call
   the deprecated OptimizeSubGraph
 * Changed the default implementation of OptimizeSubgraphView in the
   backends

Change-Id: If69903926bf5ff2aae52c9b64b4572b355662757
Signed-off-by: Matteo Martincigh <matteo.martincigh@arm.com>
diff --git a/src/backends/neon/NeonBackend.hpp b/src/backends/neon/NeonBackend.hpp
index 0d03c25..398337f 100644
--- a/src/backends/neon/NeonBackend.hpp
+++ b/src/backends/neon/NeonBackend.hpp
@@ -28,8 +28,7 @@
     IBackendInternal::Optimizations GetOptimizations() const override;
     IBackendInternal::ILayerSupportSharedPtr GetLayerSupport() const override;
 
-    IBackendInternal::SubgraphViewUniquePtr OptimizeSubgraphView(const SubgraphView& subgraph,
-                                                                 bool& optimizationAttempted) const override;
+    OptimizationViews OptimizeSubgraphView(const SubgraphView& subgraph) const override;
 };
 
 } // namespace armnn