[Docs] add Graph task for While logic

[superstar54]

The interesting thing is that the current Graph task supports the while logic out of the box. Surprise, but nice. 🚀

Here I update the doc, and a small change in the graph task api.

• In the doc control-flow, add Graph task for the While logic
• Update the decorator to allow the graph task to return a value too.
• In the Graph task execution, use a temporary workaround to fix the pickle error for the recursive call.

doc preview: https://aiida-workgraph--587.org.readthedocs.build/en/587/howto/autogen/control-flow.html#id7

[edan-bainglass]

In general, LGTM! I am a bit concerned regarding the impact on the graph visualization. Also, what is the plan to remove the pickling workaround?

[superstar54]

I am a bit concerned regarding the impact on the graph visualization.

Graph task is always shown as a black-box before running, as soon as the task runs, one can inspect it. In the case of this recursive call, the GUI will show a deeply nested workflow. (The number of nested levels is determined by the number of recursive call).

Also, what is the plan to remove the pickling workaround?

I don't know how to replace the workaround for the moment, 🤣