Is the graph symbol used in dask a general notation?
It's... a visualization. Don't read too much into it. Graphs in computer science are formalized, but unless specifically mentioned a visualization of a graph doesn't have a formal underpinning. They're generally self-explanatory, there are objects (nodes) and arrows (edges).

UML is more of the exception than the rule, being a graphical language with a formal interpretation. Most visualizations in computer science aren't that strict in meaning.

Here it seems that the round nodes are functions, the square nodes are (temporary) variables, and the arrows indicate data flow.