Use the C++ current RpcAgent pointer to eliminate the unnecessary argument passing from Python world (#32635)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32635
With the source of truth of current RPC agent moved to C++ world, there is no point of passing current RPC agent from Python world to C++ world.
ghstack-source-id: 97293316
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_process_group_debug_info
```
Differential Revision: D5703519
fbshipit-source-id: ef7c28bdb1efd293eb6cafe0b0fca7ca80fa08a6
