fix a HIP/Frontier issue (#3902)

the amrex::single_task(amrex_check_wavefront_size) check causes runtime errors

this is the check that Weiqun suggested in #3901 and I've tested that it works on Frontier with Castro

Src/Base/AMReX_GpuDevice.cpp

@@ -323,7 +323,11 @@ Device::Initialize ()
 #endif
 
 #if defined(AMREX_USE_HIP)
-    amrex::single_task(amrex_check_wavefront_size);
+    if (num_devices_used < 0) {
+        // This test is always false, but it makes the compiler no longer
+        // complain about unused function, amrex_check_wavefront_size.
+        amrex::single_task(amrex_check_wavefront_size);
+    }
 #endif
 
     Device::profilerStart();