    in each region is computed.

    Args:
        x (~chainer.Variable): Input variable. The shape is expected to be
            4 dimentional: (n: batch, c: channel, h, height, w: width).
        rois (array): Input roi. The shape is expected to
            be :math:`(R, 4)`, and each datum is set as below:
            (y_min, x_min, y_max, x_max). The dtype is :obj:`numpy.float32`.
        roi_indices (array): Input roi indices. The shape is expected to