Added const access operators

Author: ivoryw

src/autodiff.cc

@@ -130,6 +130,10 @@ double& Var::operator()(size_t x, size_t y, size_t z, size_t t){
     return data(x,y,z,t);
 }
 
+double Var::operator()(size_t x, size_t y, size_t z, size_t t) const {
+    return data(x,y,z,t);
+}
+
 Var Var::operator+(const Var& y)const{
     auto new_data = data + y.data;
     auto x_weight = nn::Tensor(data.shape, 1);

src/autodiff.hpp

@@ -33,7 +33,8 @@ class Var{
     void evaluate_leaves() const;
     nn::Tensor grad()const;
 
-    double& operator()(size_t,size_t=1,size_t=1,size_t=1);
+    double& operator()(size_t, size_t=0, size_t=0, size_t=0);
+    double operator()(size_t, size_t=0, size_t=0, size_t=0) const;
 
     Var operator+(const Var&)const;
     Var operator-(const Var&)const;