Make _wait_all_workers() support being called for multiple times (#32624)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32624
We need this PR to resolve the issue mentioned in https://github.com/pytorch/pytorch/issues/31325#issuecomment-574918917.
The solution is for each `_wait_all_workers()` call, there is a sequence ID added, to identify different calls.
ghstack-source-id: 97277591
Test Plan:
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork -- test_wait_all_workers
buck build mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork
buck-out/gen/caffe2/test/distributed/rpc/rpc_fork\#binary.par -r test_wait_all_workers
```
Differential Revision: D5739520
fbshipit-source-id: a64131e09c365179624700514422f5375afe803f
