added comment about cached device order

Signed-off-by: Byoungro So <[email protected]>

Author: bso-intel

sycl/plugins/level_zero/pi_level_zero.hpp

@@ -157,6 +157,9 @@ struct _pi_device : _pi_object {
   ze_device_handle_t ZeDevice;
 
   // Keep the subdevices that are partitioned from this pi_device for reuse
+  // The order of sub-devices in this vector is repeated from the
+  // ze_device_handle_t array that are returned from zeDeviceGetSubDevices()
+  // call, which will always return sub-devices in the fixed same order.
   std::vector<pi_device> SubDevices;
 
   // PI platform to which this device belongs.