Improve ProcessGroup RpcBackendOptions Constructor API (#34081)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34081
Before this commit, applications have to do the following to configure
number of threads in ProcessGroup RPC backend:
```
op = ProcessGroupRpcBackendOptions()
op.rpc_timeout = rpc_timeout
op.init_method = init_method
op.num_send_recv_threads = 32
init_rpc(...., rpc_backend_options=op)
```
After this commit, it can be simplified to:
```
init_rpc(...., rpc_backend_options=ProcessGroupRpcBackendOptions(num_send_recv_threads=32))
```
Fixes #34075
Test Plan: Imported from OSS
Differential Revision: D20227344
Pulled By: mrshenli
fbshipit-source-id: def4318e987179b8c8ecca44d7ff935702c8a6e7
