Suggestions for elegant_helloworld are welcome here. 给helloworld代码提建议

If you have any suggestion about `ElegantRL Helloworld`, you can discuss them here, and **we will keep an eye on this issue**.
ElegantRL's code, especially the Helloworld, really needs a lot of feedback to be better.

如果大家对ElegantRL 的 Helloworld 有什么意见，可以在此处讨论，**我们会持续关注这个问题**。
ElegantRL 的代码，尤其是 Helloworld 真的需要很多人的反馈，才能改好。

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We hope that the `ElegantRL Helloworld` would help people who want to learn about reinforcement learning to quickly run a few introductory examples.
Based on previous feedback, we've come up with a few requirements for the Helloworld code, with reasons in parentheses, to discuss with you.
- **Less lines of code**. (In order to allow the reader to read it in 10 minutes, we want the code lines <1000)
- **Less packages requirements**. (To reduce the installation time and difficulty of the code, only `torch` for deep learning, and `gym` gym[Box2D]` for RL learning environment should be installed)
- **have a gentle learning curve for reader.**. (The reader can learn about reinforcement learning through the code, or learn about the core code of ElegantRL, so we show the DRL algorithm in order: `DQN -> DDPG -> PPO`)
- **Helloworld code should be in a consistent style with the full version of ElegantRL**. (To help the reader's understanding of `ElegantRL` through `ElegantRL_helloworld`)

我们想要改写 ElegantRL 的 Helloworld 代码，希望它能帮助想要了解 强化学习的人快速运行几个入门例子。
根据大家的反馈，我们提出了对 Helloworld 代码的几点要求，并在括号内给出原因，同大家讨论：
- **代码行数要少**。（为了方便读者能在10分钟内读完，helloworld版本的代码量将维持在1000行以内）
- **代码依赖的安装的包要足够少**。（减少代码的安装时间，降低安装难度，只需要安装深度学习必备的 `torch`, 以及标准学习环境 `gym`，可选的 `gym[Box2D]`）
- **代码要有平缓的学习曲线**。（读者可以通过代码了解强化学习，或者了解ElegantRL的核心代码，所以我们按顺序展示了DRL算法：`DQN -> DDPG -> PPO`）
- **代码要与 ElegantRL 完整版的代码 保持统一的风格**。（方便读者通过 `ElegantRL_helloworld` 了解 `ElegantRL`）

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### If you are just starting learining about reinforcement learning, or coding, then your suggestions will be even more valuable to us (the helloworld code is positioned to help most people). **Feel free to leave your suggestions here.**

### 如果你对强化学习不够了解，或是对代码编写不够了解，那么你的建议对我们来说会更加宝贵（helloworld代码的定位就是要帮助大多数人）。**欢迎在这里留下你的建议。**