Add a _RemoteDevice structure for ShardedTensor/ShardingSpec. (#62927)

Add a _RemoteDevice structure for ShardedTensor/ShardingSpec. (#62927)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62927

As part of the ShardedTensor work, we realized we do need some sort of
_RemoteDevice structure that deals with our format of "workername/device" so
that users don't have to worry about parsing this string directly.

Right now this structure is just the bare minimum and is mostly a container for
describing a remote device. It is currently only used in ShardedTensor,
ShardingSpec and RemoteModule.

Once we actually have a consolidated remote device proposal, this class can be
extended appropriately if needed.
ghstack-source-id: 135534086

Test Plan:
1) unit tests
2) waitforbuildbot

Reviewed By: SciPioneer

Differential Revision: D30170689

fbshipit-source-id: 1ac2e81c7a597dc40bf3fbf2c1168c382c66649f