Fix BackendType repr in doc (#30243)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30243
Before this commit, rpc docs shows init_rpc as the following:
```
torch.distributed.rpc.init_rpc(
name,
backend=<BackendType.PROCESS_GROUP: BackendValue(
construct_rpc_agent_options_handler=<function _process_group_construct_rpc_agent_options_handler>,
init_backend_handler=<function _process_group_init_backend_handler>)>,
init_method=None,
rank=-1,
world_size=None,
rpc_agent_options=None
)
```
It unnecessarily leaks implementation details. This commit adds a
__repr__ function to BackendType Enum class to address this problem.
closes #29905
Test Plan: Imported from OSS
Differential Revision: D18641559
Pulled By: mrshenli
fbshipit-source-id: 19bf8a2d21c8207f026d097d8e3f077578d53106
