Yixing/empty tuple (#390)

* add test for empty tuple

* fix null function breaking

src/ngraph/runtime/cpu/cpu_external_function.cpp

@@ -365,6 +365,11 @@ using namespace ngraph::runtime;
     for (shared_ptr<Function> current_function : pass_manager.get_state().get_functions())
     {
         const list<shared_ptr<Node>>& tmp = current_function->get_ordered_ops();
+        if (tmp.size() < 2)
+        {
+            // Since we are comparing ops there must be at least two ops to proceed.
+            continue;
+        }
         vector<shared_ptr<Node>> op_list{tmp.begin(), tmp.end()};
         for (size_t i = 0; i < op_list.size() - 1; i++)
         {

test/builder_xla.cpp

@@ -72,3 +72,29 @@ TEST(builder_xla, simple)
     xla::call(cf, {acb}, {result_tuple});
     EXPECT_EQ((vector<float>{50, 72, 98, 128}), result->get_vector<float>());
 }
+
+TEST(builder_xla, empty_tuple_interpreter)
+{
+    auto empty_tuple = make_shared<xla::op::Tuple>(Nodes{});
+    auto f = make_shared<xla::XLAFunction>(Nodes{empty_tuple}, Nodes{});
+
+    auto manager = runtime::Manager::get("INTERPRETER");
+    auto external = manager->compile(f);
+    auto backend = manager->allocate_backend();
+    auto cf = backend->make_call_frame(external);
+
+    xla::call(cf, {}, {});
+}
+
+TEST(builder_xla, empty_tuple_cpu)
+{
+    auto empty_tuple = make_shared<xla::op::Tuple>(Nodes{});
+    auto f = make_shared<xla::XLAFunction>(Nodes{empty_tuple}, Nodes{});
+
+    auto manager = runtime::Manager::get("CPU");
+    auto external = manager->compile(f);
+    auto backend = manager->allocate_backend();
+    auto cf = backend->make_call_frame(external);
+
+    xla::call(cf, {}, {});
+}