'Args:
input: a :class:`FeedfreeInput`
devices: list of devices to be used for each training tower
nr_stage: number of elements to prefetch'

def __init__(self, input, devices, nr_stage=5):

    assert isinstance(input, FeedfreeInput), input
    self._input = input
    self._devices = devices
    self._nr_stage = nr_stage
    self._areas = []
    self._stage_ops = []
    self._unstage_ops = []
