GrpcRuntime: only send arguments to annotated placements

[mortendahl]

In the eDSL we have the placement where an argument is coming from:

def simple_computation(
    x: pm.Argument(placement=alice, vtype=pm.TensorType(pm.float64)),
    y: pm.Argument(placement=bob, vtype=pm.TensorType(pm.float64)),
):

which means we might be able to only send x to Alice, and only y to Bob, instead of currently sending everything to everyone.