'merge roidbs into one
:param a: roidb to be merged into
:param b: roidb to be merged
:return: merged imdb'

@staticmethod
def merge_roidbs(a, b):

    assert (len(a) == len(b))
    for i in range(len(a)):
        a[i]['boxes'] = np.vstack((a[i]['boxes'], b[i]['boxes']))
        a[i]['gt_classes'] = np.hstack((a[i]['gt_classes'], b[i]['gt_classes']))
        a[i]['gt_overlaps'] = np.vstack((a[i]['gt_overlaps'], b[i]['gt_overlaps']))
        a[i]['max_classes'] = np.hstack((a[i]['max_classes'], b[i]['max_classes']))
        a[i]['max_overlaps'] = np.hstack((a[i]['max_overlaps'], b[i]['max_overlaps']))
    return a
