Migrate OwnerRRef value store to generic torch Future (#38143)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38143
It's a followup of https://github.com/pytorch/pytorch/pull/32556, where an error handling boilerplate code path was added to the FutureMessage callback.
However, I noticed that the FutureMessage could never be set with an error, because the FutureMessage is a member in OwnerRRef,
- OwnerRRef does not have a setError method yet.
- The FutureMessage is only used for signaling
- The value of the RRef is contained in the `value_` field.
With the Future being generalized, it could contain more value types, not limited to Message.
This PR migrates the OwnerRRef value from the `value_` field to the generic Future.
In a later PR, it will be super easy to add a `setError` method for OwnerRRef, which calls `future_.setError(..)`. (I decide to do it later. I think it's better to migrate the call sites together with adding the new `setError` method.)
Also, this fixes the issue pointed out by https://github.com/pytorch/pytorch/pull/31086/files#r422256916.
This PR was submitted as https://github.com/pytorch/pytorch/pull/32608.
ghstack-source-id: 103757743
Test Plan:
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork
buck build mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork && \
buck-out/gen/caffe2/test/distributed/rpc/rpc_fork\#binary.par \
-r test_call_method_on_rref
```
Differential Revision: D5707692
fbshipit-source-id: 83ce0e5e5e97acb9ce8230fce5e4a3d806478b02
