Set global device back to cpu at the end of tutorial (#2411)

We are using sphinx to render those tutorials, which does not start a new process to render, so one needs to restore global state to default value, by calling `torch.set_default_device('cpu')`

Co-authored-by: Svetlana Karslioglu <svekars@fb.com>

Author: malfet

recipes_source/recipes/changing_default_device.py

@@ -43,6 +43,9 @@
 print(mod.weight.device)
 print(mod(torch.randn(128, 20)).device)
 
+# And then globally return it back to CPU
+torch.set_default_device('cpu')
+
 ################################################################
 # This function imposes a slight performance cost on every Python
 # call to the torch API (not just factory functions). If this