[pytorch] Minor: add GIL assert to PythonRpcHandler::handleExceptionGILHeld (#33557)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33557
We should add GIL asserts in some places to keep assumptions documented.
This just adds one in an exception codepath as a placeholder for more.
This change also moves a #define from a .h to the .cpp to reduce scope.
ghstack-source-id: 98673532
Test Plan: buck test mode/dev-nosan caffe2/test/...
Differential Revision: D20005387
fbshipit-source-id: b7eff54a6f1dd69d199f8ca05cdb3001c50b37c4
