    def __call__(self, video_len, max_video_len, center):
        """
        `video_len`: length of the video.
        `max_video_len`: maximum video tokens allowd in a sequence.
        `center`: initial starting index.
        """
        assert center >= 0 and center < video_len
        t_clip_len = 0
        start, end = center, center
        while (start > 0 or end < video_len) and t_clip_len < max_video_len:
            # decide the direction to grow.
            if start <= 0:
                end += 1
            elif end >= video_len:
                start -= 1
            elif random.random() > 0.5:
                end += 1
            else:
                start -= 1
            t_clip_len += 1
        return {"start": [start], "end": [end]}
`video_len`: length of the video.
`max_video_len`: maximum video tokens allowd in a sequence.
`center`: initial starting index.
examples/MMPT/mmpt/processors/how2processor.py