'Args:
get_tensor_fn: a function which returns a list of input tensors
when called. It will be called under a TowerContext.
size(int): size of this input. Use None to leave it undefined.'

def __init__(self, get_tensor_fn, size=None):

    self.get_tensor_fn = get_tensor_fn
    if (size is not None):
        size = int(size)
        assert (size > 0)
    self._fixed_size = size
