Fixed warning in visual studio.

dlib/dnn/tensor.h

@@ -168,7 +168,7 @@ namespace dlib
             const matrix_exp<EXP>& item
         )
         {
-            DLIB_CASSERT(idx < num_samples());
+            DLIB_CASSERT(idx < (unsigned long)num_samples());
             DLIB_CASSERT(item.size() == nr()*nc()*k());
             static_assert((is_same_type<float, typename EXP::type>::value == true),
                 "To assign a matrix to a tensor the matrix must contain float values");