[torch/elastic] Expose a `stderr` parameter in `EtcdServer`. (#54805)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54805
Expose a `stderr` parameter to `EtcdServer` to have a clean unit test outputs.
Test Plan: Run the existing test suite.
Reviewed By: kiukchung
Differential Revision: D27327495
fbshipit-source-id: 0a342aeda0ff4d85d809aab1cbf155d3fafd4fa1
