Understanding different modes of the Driver

Firstly, congrats on the repo!

I am trying better understand the framework. Can you explain a bit more about the different modes "executor", "scheduler", "ray". https://github.com/deepseek-ai/smallpond/blob/main/smallpond/execution/driver.py#L32

I have seen text on "ray" and "scheduler" mode in other places but not sure about the "executor" mode, especially since that is the default option used in the benchmarks.

cc. [wangrunji0408](https://github.com/wangrunji0408)
Thanks!