📚 Documentation

Per torch.distributed.rpc.init_model_parallel function on RPC document page in master branch here, I think there are some points to revise or polish:

1. the argument worker_name_to_id is missing in doc.
2. The order of arguments in doc does not match the function signature, which would cause additional ambiguity or inconvenience for readers.
3. I think the repr for argument backend is too long to read, which may contain a lot of unneeded information for readers. It is arguable but it might be better to give the class a concise repr.

cc @pietern @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @xush6528