Implement backend-agnostic rpc._wait_all_workers() utility (#31888)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31888
We need a backend-agnostic mechanism to do barrier-like operation before locally destroy RRef context and shutdown RPC Agent.
- Sort worker names.
- Elect the first name as the leader in the ordered worker names.
- Followers reports therir intent to synchronize to the leader.
- Leader also reports to itself, when `_wait_all_workers()` called.
- If all workers report their intent to proceed, leader send the command to every one to proceed.
ghstack-source-id: 96386210
Test Plan:
# Unit tests
```
buck test mode/dev-nosan //caffe2/test:rpc_fork
buck-out/gen/caffe2/test/rpc_fork\#binary.par -r test_wait_all_workers
buck-out/gen/caffe2/test/rpc_fork\#binary.par -r test_rref_leak
```
```
buck test mode/dev-nosan //caffe2/test:rpc_fork_thrift
buck-out/gen/caffe2/test/rpc_fork\#binary.par -r test_wait_all_workers
buck-out/gen/caffe2/test/rpc_fork_thrift\#binary.par -r test_worker_id
```
# Stress runs
```
buck test mode/dev-nosan //caffe2/test:rpc_fork_thrift -- test_stress_light_rpc --stress-runs 10
```
```
buck test mode/dev-nosan //caffe2/test:rpc_spawn_thrift -- test_stress_light_rpc --stress-runs 10
```
```
buck test mode/dev-nosan //caffe2/test:rpc_fork_thrift -- test_stress_heavy_rpc --stress-runs 10
```
```
buck test mode/dev-nosan //caffe2/test:rpc_spawn_thrift -- test_stress_heavy_rpc --stress-runs 10
```
Differential Revision: D19290954
fbshipit-source-id: cdb22203c2f27b5e0d0ad5b2d3b279d438c22dcf
