Delete all user forks tracked in RRefContext before graceful shutting down (#31893)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31893
In order to resolve the issue summarized in https://github.com/pytorch/pytorch/issues/31325.
The overal solution is to proactively send out delete fork messages from user nodes, before user nodes detecting rref leaks.
As the first step, we want to have a weak ref tracker to track all user rrefs.
ghstack-source-id: 100023142
Test Plan:
V22 is the version that make User to wait on delete UseerRRef message.
# Unit tests
```
buck test mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork
buck test mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork -- test_nested_rref_stress --stress-runs 100
buck build mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork \
&& buck-out/gen/caffe2/test/distributed/rpc/rpc_fork\#binary.par -r test_nested_rref_stress
buck build mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork \
&& buck-out/gen/caffe2/test/distributed/rpc/rpc_fork\#binary.par - r test_rref_forward_chain
buck build mode/dev-nosan //caffe2/test/distributed/rpc:rpc_fork \
&& buck-out/gen/caffe2/test/distributed/rpc/rpc_fork\#binary.par -r test_non_garbage_collected_user_rref_due_to_local_circular_dependency
```
Reviewed By: mrshenli
Differential Revision: D19292254
fbshipit-source-id: 92c3e8d0b00f183c5e22f163bdca482cc25a1ce9
