[Bug]: [question generation] template 3 NameError: name 'tokenizer' is not defined

[shm007g]

软件环境

- paddlepaddle: 2.3.2
- paddlepaddle-gpu: None
- paddlenlp: 2.4.2

重复问题

• I have searched the existing issues

错误描述

\paddlenlp\taskflow\question_generation.py", line 194, in _convert_example

source = '这是一个问题生成任务，根据提供的答案和上下文，来生成问题。' + title + tokenizer.sep_token + '上下文：' + source
NameError: name 'tokenizer' is not defined

这里代码有问题，template 3 写了一个没定义的tokenizer。unittest要跟进啊百度gg们。

### 稳定复现步骤 & 代码

Taskflow(
    "question_generation",
    model='unimo-text-1.0-question-generation-dureader_qg',
    # model='unimo-text-1.0-question-generation',
    # model='unimo-text-1.0-dureader_qg',
    # model='unimo-text-1.0',
    template=3,
    device='cpu',
    min_length=1,
    num_beams=15,
    num_return_sequences=10,
    is_select_from_num_return_sequences=False,
    output_scores=True,
)

[westfish]

bug已经修复，请参考#3646