渲染有bug，dropout掉的内容未从标签中剔除

core/layout/__init__.py中添加：
waiting_removing = []    # 添加
for block in self.block_list:
    if np.random.rand() < dropout:
        log.info('dropout incurred. ')
        waiting_removing.append(block)    # 添加
        continue

block渲染完后删除dropout掉的标签：
for wr in waiting_removing:
    self.block_list.remove(wr)