add virtual method to get version number from Backend (#3485)

* add virtual method to get version number from Backend

* style

* semicolon

src/ngraph/runtime/backend.hpp

@@ -174,6 +174,9 @@ public:
 
     virtual bool executable_can_create_tensors();
 
+    /// \brief Get the version of the backend
+    /// The default value of 0.0.0 is chosen to be a parsable version number
+    virtual std::string get_version() const { return "0.0.0"; }
 private:
     // mutex to modify s_backend_shared_library_search_directory thread safe
     static std::mutex m_mtx;