[ONNX] Use v1 ops in Softplus (#4092)

* Use v1 ops in ONNX Softplus

* Some extra comments
Co-authored-by: Scott Cyphers <diyessi@users.noreply.github.com>

src/ngraph/frontend/onnx_import/op/softplus.cpp

@@ -18,10 +18,6 @@
 
 #include "default_opset.hpp"
 #include "ngraph/node.hpp"
-#include "ngraph/op/add.hpp"
-#include "ngraph/op/greater.hpp"
-#include "ngraph/op/select.hpp"
-#include "ngraph/opsets/opset0.hpp"
 #include "softplus.hpp"
 
 namespace ngraph
@@ -34,34 +30,38 @@ namespace ngraph
             {
                 NodeVector softplus(const Node& node)
                 {
-                    auto data = node.get_ng_inputs().at(0);
+                    const auto data = node.get_ng_inputs().at(0);
 
-                    std::shared_ptr<ngraph::Node> zero_node =
+                    const std::shared_ptr<ngraph::Node> zero_node =
                         std::make_shared<default_opset::Constant>(
                             data->get_element_type(), data->get_shape(), std::vector<float>{0.f});
-                    std::shared_ptr<ngraph::Node> one_node =
+                    const std::shared_ptr<ngraph::Node> one_node =
                         std::make_shared<default_opset::Constant>(
                             data->get_element_type(), data->get_shape(), std::vector<float>{1.f});
 
-                    std::shared_ptr<ngraph::Node> positive_val_node =
-                        data + std::make_shared<default_opset::Log>(
-                                   std::make_shared<default_opset::Exp>(
-                                       std::make_shared<default_opset::Negative>(data)) +
-                                   one_node);
+                    // data + log(exp(-data) + 1)
+                    const std::shared_ptr<ngraph::Node> positive_val_node =
+                        std::make_shared<default_opset::Add>(
+                            data,
+                            std::make_shared<default_opset::Log>(
+                                std::make_shared<default_opset::Add>(
+                                    std::make_shared<default_opset::Exp>(
+                                        std::make_shared<default_opset::Negative>(data)),
+                                    one_node)));
 
-                    std::shared_ptr<ngraph::Node> negative_val_node =
-                        std::make_shared<default_opset::Log>(
-                            std::make_shared<default_opset::Exp>(data) + one_node);
+                    // log(exp(data) + 1)
+                    const std::shared_ptr<ngraph::Node> negative_val_node =
+                        std::make_shared<default_opset::Log>(std::make_shared<default_opset::Add>(
+                            std::make_shared<default_opset::Exp>(data), one_node));
 
-                    std::shared_ptr<ngraph::Node> condition_node =
-                        std::make_shared<ngraph::opset0::Greater>(data, zero_node);
+                    const std::shared_ptr<ngraph::Node> condition_node =
+                        std::make_shared<default_opset::Greater>(data, zero_node);
 
-                    //
                     // This equation represents:
                     //     x + log(exp(-x) + 1) - for x > 0; to manage exponent overflow,
                     //     log(exp(x) + 1)      - elsewhere.
                     //
-                    return {std::make_shared<ngraph::opset0::Select>(
+                    return {std::make_shared<default_opset::Select>(
                         condition_node, positive_val_node, negative_val_node)};
                 }