Made non-const alias tensors convertible to const alias tensors.

dlib/dnn/tensor.h

@@ -566,6 +566,8 @@ namespace dlib
         const tensor& get() const { return inst; }
         operator const tensor& () { return inst; }
 
+        alias_tensor_const_instance(const alias_tensor_instance& item) : inst(item) {}
+
     private:
         alias_tensor_instance inst;
 

dlib/dnn/tensor_abstract.h

@@ -601,6 +601,9 @@ namespace dlib
 
     public:
 
+        // non-const alias tensors are convertible to const ones.
+        alias_tensor_const_instance(const alias_tensor_instance& item); 
+
         // Methods that cast the alias to a tensor.
         const tensor& get() const;
         operator const tensor& ();