Let rpc._all_gather use default RPC timeout (#44983)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44983
`_all_gather` was converted from `_wait_all_workers` and inherited its
5 seconds fixed timeout. As `_all_gather` meant to support a broader
set of use cases, the timeout configuration should be more flexible.
This PR makes `rpc._all_gather` use the global default RPC timeout.
Test Plan: Imported from OSS
Reviewed By: pritamdamania87
Differential Revision: D23794383
Pulled By: mrshenli
fbshipit-source-id: 382f52c375f0f25c032c5abfc910f72baf4c5ad9
