Execution plan of the flow. This is an array of sets containing integer elements.
The integer elements refer to index of a task defined in
crate::model::Flow.
A task is executed in multiple stages, where each stage is a set of tasks.
The set of tasks in the last stage are dependent set of tasks in the last but second stage and so on,
with the first stage having tasks that are independent having no dependencies (leaf tasks).
Tasks belonging to the same stage are not dependent on each other, if a task is dependent on another task,
they will belong to different stages. The number of tasks in each stage need not be equal.