Function relay_schedule 

pub fn relay_schedule(
    chips: &[SpatialProgram],
    transfers: &[ChipTransfer],
) -> Result<Vec<RelayStep>, String>

The host’s relay over chips’ halts: round by round, each chip that can is run to its next halt — once the tensors the halt it stands at receives have been sent and copied — so chips whose next work needs nothing of each other run at once. Refused, with where each chip stands, when no chip can move before all have finished.