Add retry logic for test_multitenancy and documentation for find_free_port (#67775)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67775
cc pietern mrshenli pritamdamania87 zhaojuanmao satgera rohan-varma gqchen aazzolini osalpekar jiayisuse SciPioneer H-Huang
Test Plan: Imported from OSS
Reviewed By: rohan-varma
Differential Revision: D32142749
Pulled By: H-Huang
fbshipit-source-id: 67ab4ede4f4bff96a1ffd41d55b3be0edc82b1ce
