About keeping `elegantrl_helloworld` simple.

我们想让 `elegantrl_helloworld` 容易安装、运行、阅读。
We want to keep `elegantrl_helloworld` easy to install/ run/ read.

- **容易安装**：只需安装 `torch` 和 `gym`
- **容易运行**：只有四个核心文件 `agent.py, net.py, run.py, config.py`
- **容易阅读**：总代码行数不超过1000行

- **easy to install**: only need to install `torch` and `gym`
- **easy to run**: only 4 kernel files: `agent.py, net.py, run.py, config.py`
- **easy to read**: the total code number less than 100 lines.

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举个例子：`yaml` 更适合放在 完整版的`elegantrl`里，但不适合放在 `elegantrl_hellowworld`里
For example: `yaml` fits better in the full version of `elegantrl`, but not in `elegantrl_hellowworld`.