Add constant version of IConnectableLayer::GetConstantTensorsByRef

This makes it easier to use, particularly in backends where it
is common to pass around const pointers to IConnectableLayer.

The non-constant version is rewritten to use the constant version.

Signed-off-by: Matthew Bentham <matthew.bentham@arm.com>
Change-Id: Id3a8384447e93c213299a85ade9a667df5960534
diff --git a/delegate/src/MultiLayerFacade.hpp b/delegate/src/MultiLayerFacade.hpp
index aa00be8..31a7354 100644
--- a/delegate/src/MultiLayerFacade.hpp
+++ b/delegate/src/MultiLayerFacade.hpp
@@ -123,6 +123,7 @@
     /// Retrieve the handles to the constant values stored by the layer.
     /// @return A vector of the constant tensors stored by this layer.
     ConstantTensors GetConstantTensorsByRef() override { return {}; }
+    ImmutableConstantTensors GetConstantTensorsByRef() const override { return {}; }
 
 private:
     armnn::IConnectableLayer* m_FirstLayer;