[Flaky tests] Fix test_all_gather_timeout test (#45989)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45989
This test was failing internally for the Thrift-based RPC agent, since
it has a different error regex. Use `self.get_timeout_error_regex` which gets
the timeout error string for each backend to fix this.
ghstack-source-id: 114463458
Test Plan: CI
Reviewed By: pritamdamania87
Differential Revision: D24170394
fbshipit-source-id: 9b30945e3e30f36472268d042173f8175ad88098
