[torchelastic][rendezvous] Add option to enable libuv for TCPStore based rendezvous backend (#118944)
Summary:
Expose an option to enable libuv in TCPStore based rendezvous backend that will allow better scaling.
Libuv support has been added recently and allows scaling for more than 2K nodes.
Test Plan: Unit tests
Differential Revision: D53335860
Pull Request resolved: https://github.com/pytorch/pytorch/pull/118944
Approved by: https://github.com/wconstab
