[DOCS] Added an evaluate method for custom operation (#2273)

* Added an evaluate method for custom operation

* Fixed comments

docs/IE_DG/Extensibility_DG/AddingNGraphOps.md

@@ -16,6 +16,8 @@ To add your custom nGraph operation, create a new class that extends `ngraph::Op
 
 5. Override the `visit_attributes` method, which allows serialization and deserialization of attributes. An `AttributeVisitor` is passed to the method, and the implementation is expected to walk over all the attributes in the op using the type-aware `on_attribute` helper. Helpers are already implemented for standard C++ types like `int64_t`, `float`, `bool`, `vector` and for existing nGraph defined types.
 
+6. Override `evaluate`, which is an optional method that enables the application of constant folding if there is a custom operation on the constant branch.
+
 Based on that, declaration of a operation class can look as follows:
 
 @snippet op.hpp op:header
@@ -51,6 +53,12 @@ nGraph operation contains two constructors: a default constructor, which allows
 
 @snippet op.cpp op:visit_attributes
 
+### `evaluate()`
+
+`ngraph::Node::evaluate` method allows to apply constant folding to an operation.
+
+@snippet op.cpp op:evaluate
+
 ## Register Custom Operations in Extension Class
 
 To add custom operations to the [Extension](Extension.md) class, create an operation set with custom operations and implement the `InferenceEngine::IExtension::getOpSets` method:

docs/template_extension/op.cpp

@@ -36,3 +36,52 @@ bool Operation::visit_attributes(ngraph::AttributeVisitor &visitor) {
     return true;
 }
 //! [op:visit_attributes]
+
+//! [op:evaluate]
+namespace
+{
+
+template <class T>
+void implementation(const T* input,
+                    T* output,
+                    int64_t add,
+                    size_t size) {
+    for (size_t i = 0; i < size; i++) {
+        output[i] = input[i] + add;
+    }
+}
+
+template <ngraph::element::Type_t ET>
+bool evaluate_op(const ngraph::HostTensorPtr& arg0,
+                 const ngraph::HostTensorPtr& out, int64_t add)
+{
+    size_t size = ngraph::shape_size(arg0->get_shape());
+    implementation(arg0->get_data_ptr<ET>(),
+                   out->get_data_ptr<ET>(),
+                   add,
+                   size);
+    return true;
+}
+
+}  // namespace
+
+bool Operation::evaluate(const ngraph::HostTensorVector& outputs,
+                         const ngraph::HostTensorVector& inputs) const {
+    switch (inputs[0]->get_element_type())
+    {
+    case ngraph::element::Type_t::i8: return evaluate_op<ngraph::element::Type_t::i8>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::i16: return evaluate_op<ngraph::element::Type_t::i16>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::i32: return evaluate_op<ngraph::element::Type_t::i32>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::i64: return evaluate_op<ngraph::element::Type_t::i64>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::u8: return evaluate_op<ngraph::element::Type_t::u8>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::u16: return evaluate_op<ngraph::element::Type_t::u16>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::u32: return evaluate_op<ngraph::element::Type_t::u32>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::u64: return evaluate_op<ngraph::element::Type_t::u8>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::bf16: return evaluate_op<ngraph::element::Type_t::bf16>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::f16: return evaluate_op<ngraph::element::Type_t::f16>(inputs[0], outputs[0], getAddAttr());
+    case ngraph::element::Type_t::f32: return evaluate_op<ngraph::element::Type_t::f32>(inputs[0], outputs[0], getAddAttr());
+    default: break;
+    }
+    return false;
+}
+//! [op:evaluate]

docs/template_extension/op.hpp

@@ -19,7 +19,9 @@ public:
     void validate_and_infer_types() override;
     std::shared_ptr<ngraph::Node> clone_with_new_inputs(const ngraph::OutputVector& new_args) const override;
     bool visit_attributes(ngraph::AttributeVisitor& visitor) override;
-    int64_t getAddAttr() { return add; }
+    int64_t getAddAttr() const { return add; }
+    bool evaluate(const ngraph::HostTensorVector& outputs,
+                  const ngraph::HostTensorVector& inputs) const override;
 
 private:
     int64_t add;