Mark prim::rpc_async as having side effect (#35994)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35994
prim::rpc_async was optimized out if we don't takes its returned future and wait on the future.
Test Plan: `
Differential Revision: D7850846
fbshipit-source-id: e4e46506ab608f2e072027d6c10c49a4d784b14a
