Refactored Optimize(...) function to throw exceptions instead of returning null

 * INetwork::Optimize(...) states that the function should throw an exception
   if it fails but the implementation in Network.cpp returned null in some
   scenarios instead. This has led to some confusion amongst users.

Signed-off-by: Mike Kelly <mike.kelly@arm.com>
Change-Id: I358d1293232c9464772aa0e39ab3355e3570c823
diff --git a/src/backends/backendsCommon/test/OptimizedNetworkTests.cpp b/src/backends/backendsCommon/test/OptimizedNetworkTests.cpp
index d82ee0e..c8986f5 100644
--- a/src/backends/backendsCommon/test/OptimizedNetworkTests.cpp
+++ b/src/backends/backendsCommon/test/OptimizedNetworkTests.cpp
@@ -84,8 +84,18 @@
     armnn::IRuntimePtr runtime(armnn::IRuntime::Create(options));
 
     std::vector<armnn::BackendId> backends = { armnn::Compute::CpuAcc };
-    armnn::IOptimizedNetworkPtr optNet = armnn::Optimize(*net, backends, runtime->GetDeviceSpec());
-    BOOST_CHECK(!optNet);
+    std::vector<std::string> errMessages;
+
+    try
+    {
+        Optimize(*net, backends, runtime->GetDeviceSpec(), armnn::OptimizerOptions(), errMessages);
+        BOOST_FAIL("Should have thrown an exception.");
+    }
+    catch (const armnn::InvalidArgumentException& e)
+    {
+        // Different exceptions are thrown on different backends
+    }
+    BOOST_CHECK(errMessages.size() > 0);
 }
 
 BOOST_AUTO_TEST_CASE(OptimizeValidateDeviceNonSupportLayerWithFallback)
@@ -190,10 +200,18 @@
     armnn::IRuntimePtr runtime(armnn::IRuntime::Create(options));
 
     std::vector<armnn::BackendId> backends = { armnn::Compute::Undefined };
+    std::vector<std::string> errMessages;
 
-    armnn::IOptimizedNetworkPtr optNet = armnn::Optimize(net, backends, runtime->GetDeviceSpec());
-    BOOST_CHECK(!optNet);
-
+    try
+    {
+        Optimize(net, backends, runtime->GetDeviceSpec(), armnn::OptimizerOptions(), errMessages);
+        BOOST_FAIL("Should have thrown an exception.");
+    }
+    catch (const armnn::InvalidArgumentException& e)
+    {
+        // Different exceptions are thrown on different backends
+    }
+    BOOST_CHECK(errMessages.size() > 0);
 }
 
 BOOST_AUTO_TEST_CASE(OptimizeValidateWorkloadsUndefinedComputeDeviceWithFallback)