use gloo enum instead of hardcoding stirng (#27652)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27652
Changes "gloo" to dist.backend.GLOO in rpc_test.py.
ghstack-source-id: 91764460
Test Plan: python test/test_rpc_fork.py && python test/test_rpc_spawn.py
Differential Revision: D17845067
fbshipit-source-id: b220d3672d1e0b237da474276663d157230a4fdb
