[rpc] Move _set_rpc_backand and RpcBackendOptions to use float instead of timedelta (#37027)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37027
The RPC timeout passed into rpc_sync and rpc_async after the below
change is now float, so we should make these APIs consistent.
ghstack-source-id: 102971906
Test Plan:
Existing unittests, also added unittest testing specific timeout set
in ProcessGroupRpcBackendOptions and the dispatch rpc backend options handling.
Differential Revision: D21125171
fbshipit-source-id: a5894b8ce31d2926f2c3d323d1cda4d54b30cef1
