build a generic future<T> (#29579)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29579
Per #28923, this diff is to move Future<Message> to torch::utils and extend it to be Future<T>, most of implementations are copied from FutureMessage and ivalue::Future. merge ivalue::Future with Future<T> will be done separately.
The main difference between Future<T> and FutureMessage is the error handling, instead of checking message type inside Future to handle error, this future<T> owns has_error_ and error_ states.
also this future passes value_, has_error_ and error_ states to callbacks for easily read future states.
In next diff, a torch script rpc async API will be created, before the API returns, it will create an ivalue::Future and passes it to Future<T>'s call back where state of ivalue::Future will be set. In this way, the torch script rpc async API can still return a ivalue::Future and call wait() to get its state appropriately afterwards.
ghstack-source-id: 95479525
Test Plan: unit tests
Differential Revision: D18263023
fbshipit-source-id: 48a65712656a72c2feb0bb3ec8b308c0528986a6
