add node_type enum

scalar.h

@@ -20,15 +20,25 @@ class Scalar: public std::enable_shared_from_this<Scalar<T>> {
     }
 
 public:
+    enum class NodeType {
+        INPUT,
+        WEIGHT,
+        COMPUTED
+    };
+
+
+    const NodeType node_type;
     const int id = id_counter++;
     T value, grad = 0;
     int in_degrees = 0;
 
     std::set<std::shared_ptr<Scalar<T>>> children;
     std::function<void()> _backward;
 
-    Scalar() : Scalar(0) {}
-    Scalar(T value) : value{value}, in_degrees{0} { _backward = []() {}; }
+    Scalar() : Scalar(0, NodeType::COMPUTED) {}
+    Scalar(T value, NodeType node_type) : value{value}, in_degrees{0}, node_type{node_type} { _backward = []() {}; }
+    Scalar(T value) : Scalar(value, NodeType::COMPUTED) {}
+
 
     static std::shared_ptr<Scalar<T>> make(T value) {
         auto s = std::make_shared<Scalar<T>>(value);