Added a checklist item in PR template for GPU and CPU testing

Signed-off-by: Kieran Fraser <Kieran.Fraser@ibm.com>

Author: kieranfraser

PULL_REQUEST_TEMPLATE.md

@@ -36,3 +36,4 @@ Please describe the tests that you ran to verify your changes. Consider listing
 - [ ] My changes generate no new warnings
 - [ ] I have added tests that prove my fix is effective or that my feature works
 - [ ] New and existing unit tests pass locally with my changes
+- [ ] My changes have been tested using both CPU and GPU devices