Abstract torch.distributed APIs in CheckpointLoader

[g-husam]

Consume abstract function getters for get_rank/get_local_rank, and a protocol for collective comms APIs (like broadcast_object), to make it easier to test and swap impls via dependency injection.

See DefaultMLFlashpointCheckpointSaver as an example, to do the same for other classes like DefaultMLFlashpointCheckpointLoader.

[ronaldw07]

I've opened a PR to address this: #47