Made alias_tensor usable when it is const.

dlib/dnn/tensor.h

@@ -615,7 +615,7 @@ namespace dlib
         alias_tensor_instance operator() (
             tensor& t,
             size_t offset
-        ) 
+        ) const
         {
             DLIB_CASSERT(offset+size() <= t.size());
 
@@ -637,7 +637,7 @@ namespace dlib
         alias_tensor_const_instance operator() (
             const tensor& t,
             size_t offset
-        )
+        ) const
         {
             alias_tensor_const_instance temp;
             temp.inst = (*this)(const_cast<tensor&>(t),offset);
@@ -645,7 +645,7 @@ namespace dlib
         }
 
     private:
-        alias_tensor_instance inst;
+        mutable alias_tensor_instance inst;
     };
 
     inline void serialize(const alias_tensor& item, std::ostream& out)

dlib/dnn/tensor_abstract.h

@@ -663,7 +663,7 @@ namespace dlib
         alias_tensor_instance operator() (
             tensor& t,
             size_t offset
-        );
+        ) const;
         /*!
             requires
                 - offset+size() <= t.size()
@@ -684,7 +684,7 @@ namespace dlib
         alias_tensor_const_instance operator() (
             const tensor& t,
             size_t offset
-        );
+        ) const;
         /*!
             requires
                 - offset+size() <= t.size()