Return a list of assignment operations that syncs the parameters
    of all model copies with the one on host_device.
    :param host_device: (required str) the name of the device with latest
                        parameters

def create_sync_ops(self, host_device):
    """
    Return a list of assignment operations that syncs the parameters
    of all model copies with the one on host_device.
    :param host_device: (required str) the name of the device with latest
                        parameters
    """
    host_device = unify_device_name(host_device)
    sync_ops = []
    for layer in self.layers:
      if isinstance(layer, LayernGPU):
        sync_ops += layer.create_sync_ops(host_device)
    return sync_ops