Add `LaunchResult`

[apoorvkh]

We should wrap the return values in a class called LaunchResult (or similar). This is rather important, because it is pretty ugly / unintuitive to value = trx.launch("...")["localhost"][0] or equivalent. Especially if you don't know the node names (in "auto" mode) or don't know which is rank zero.

Support functions .all(by: Literal['rank', 'hostname']) (all results, ordered by global rank or hostname+local), .value(rank: int) (global rank), .value(hostname: str) (list of values from specific host), .value(hostname: str, rank: int) (local rank)

[apoorvkh]

Does AgentStatus.return_values need to be dict[int, Any | WorkerException]? If it is indexed by local_rank, then I guess it can just be list[Any | WorkerException]

[apoorvkh]

Started https://github.com/apoorvkh/torchrunx/tree/launch-result