Add multi-gpu inference option

[natolambert]

Currently run_rm.py only uses one RM because RMs are not well supported generally for inference.
Current implementation is a separate run_rm_mpgu.py script. We can delete this and improve the base script if more use cases emerge.

Closes #95

[natolambert]

Inference works distributed, but I couldn't get the results gather working correctly with things like

    state.wait_for_everyone()
    
    # flatten results list of lists if is list of lists
    if state.is_main_process:
        logger.info("Gathering results")
        results = gather_object(results) # gather() is for tensors