[STF] Declare from_index at namespace scope for unqualified lookup

The friend definition inside data_place is only found via ADL when a
data_place argument is present. Calls like from_index(n) in logical_data.cuh
have no data_place argument, so add a matching declaration at namespace
scope so the identifier is visible to unqualified lookup.

Made-with: Cursor

Author: andralex

cudax/include/cuda/experimental/__stf/places/places.cuh

@@ -388,6 +388,9 @@ private:
   ::std::shared_ptr<data_place_interface> pimpl_;
 };
 
+/** Declaration for unqualified lookup (friend is only found via ADL when a \c data_place argument is present). */
+inline data_place from_index(size_t n);
+
 /**
  * @brief Indicates where a computation takes place (CPU, dev0, dev1, ...)
  *