'Sets the static shape of the target.
Args:
target: the target tensor. Its first dimension will be overwritten.
num_anchors: the number of anchors, which is used to override the target\'s
first dimension.
Returns:
A tensor with the shape info filled in.'

def _reset_target_shape(self, target, num_anchors):

    target_shape = target.get_shape().as_list()
    target_shape[0] = num_anchors
    target.set_shape(target_shape)
    return target
