add get_preferred_pipeline_depth to executable (#3687)

* add get_default_pipeline_depth to executable

* use get_preferred_pipeline_depth

src/ngraph/runtime/executable.cpp

@@ -108,6 +108,11 @@ const ngraph::ResultVector& runtime::Executable::get_results() const
     return m_results;
 }
 
+size_t runtime::Executable::get_preferred_pipeline_depth() const
+{
+    return 2;
+}
+
 void runtime::Executable::set_parameters_and_results(const Function& func)
 {
     m_parameters = func.get_parameters();

src/ngraph/runtime/executable.hpp

@@ -69,6 +69,10 @@ public:
     /// \returns an ngraph::ResultVector of all input parameters
     const ngraph::ResultVector& get_results() const;
 
+    /// \brief Get the preferred pipeline_depth for this executable
+    /// \returns  preferred pipeline_depth
+    virtual size_t get_preferred_pipeline_depth() const;
+
     /// \brief Save this compiled Executable to an output stream.
     ///    Saved stream may be read with Backend::load
     virtual void save(std::ostream& output_stream);