[DDP] Log num threads (#64072)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64072
Log gloo threads to DDP logging.
ghstack-source-id: 137119480
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D30596083
fbshipit-source-id: 2b4f6e762cb5d850be6056bcc5922029a1af3c91
