Add RpcAgent::getWorkerInfos() (#30241)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30241
We need an API to get all worker infos. This will be used by backend-agnostic `rpc.wait_all_workers()` API.
ghstack-source-id: 94454935
Test Plan:
# Unit tests
```
buck test mode/dev-nosan //caffe2/test:rpc_fork -- test_get_worker_infos
buck-out/gen/caffe2/test/rpc_fork\#binary.par -r test_get_worker_infos
```
```
buck test mode/dev-nosan //caffe2/test:rpc_fork_thrift -- test_get_worker_infos
buck-out/gen/caffe2/test/rpc_fork_thrift\#binary.par -r test_get_worker_infos
```
Differential Revision: D5693412
fbshipit-source-id: 5123c8248b6d44fd36b8a5f381dbabb2660e6f0f
