add visitor attribute test case

ngraph/test/CMakeLists.txt

@@ -264,6 +264,7 @@ set(SRC
     visitors/op/roi_pooling.cpp
     visitors/op/shuffle_channels.cpp
     visitors/op/softmax.cpp
+    visitors/op/softplus.cpp
     visitors/op/space_to_depth.cpp
     visitors/op/split.cpp
     visitors/op/squared_difference.cpp

ngraph/test/visitors/op/softplus.cpp

@@ -0,0 +1,25 @@
+// Copyright (C) 2021 Intel Corporation
+// SPDX-License-Identifier: Apache-2.0
+//
+
+#include "gtest/gtest.h"
+#include "ngraph/ngraph.hpp"
+#include "ngraph/opsets/opset1.hpp"
+#include "util/visitor.hpp"
+
+using namespace std;
+using namespace ngraph;
+using ngraph::test::NodeBuilder;
+using ngraph::test::ValueMap;
+
+TEST(attributes, softplus)
+{
+    NodeBuilder::get_ops().register_factory<opset1::Squeeze>();
+    const auto data_node = make_shared<op::Parameter>(element::f32, Shape{1});
+    const auto result = make_shared<op::v4::SoftPlus>(data_node);
+
+    NodeBuilder builder(result);
+    const auto expected_attr_count = 0;
+
+    EXPECT_EQ(builder.get_value_map_size(), expected_attr_count);
+}