[ez] [Docs] Missing import in example for post_local_sgd (#67047)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67047
Fix missing import
ghstack-source-id: 141258423
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D31841837
fbshipit-source-id: 139e614517dcac7a53259ff7a0360bb5275bb53b
