minor: change device selection in poststack_cupy.py

Co-authored-by: Rohan Babbar  <[email protected]>

Author: mrava87

tutorials_cupy/poststack_cupy.py

@@ -20,7 +20,8 @@
 
 plt.close("all")
 rank = MPI.COMM_WORLD.Get_rank()
-cp.cuda.Device(device=rank).use();
+device_count = cp.cuda.runtime.getDeviceCount()
+cp.cuda.Device(rank % device_count).use()
 
 ###############################################################################
 # Let's start by defining all the parameters required by the