[RPC] Add to confirmed users immediately if the fork is shared from owner, instead of adding nothing to pending users (#34988)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34988
In https://github.com/pytorch/pytorch/pull/31893, we introduced a confirmedUsers_ map in RRefContext.
For the case that the fork is shared from the owner, there is no `pendingUsers_` intermediate phase for this fork, we should put this fork into `confirmedUsers_` immediately.
Test Plan:
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork
```
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc/jit:rpc_fork
```
Differential Revision: D7735909
fbshipit-source-id: 14c36a16486f0cc9618dcfb111fe5223781b647d
