[WIP] Add topological sort algorithm

[ngokulakrish]

This PR is to add topological sort algorithm to cuGraph.

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[seunghwak]

Looks great except for minor cosmetic issues! We need to add tests and work on merging this.

[seunghwak]

This doesn't belong to the traversal group.

https://github.com/rapidsai/cugraph/blob/main/cpp/include/cugraph/algorithms.hpp#L31

We may create dag_group here. That place this algorithm in the dag_group.

@ChuckHastings @acostadon @alexbarghi-nv @rlratzel @BradReesWork Any thoughts on where should we place topological sort?

In networkX, topological sort is in dag.py (https://github.com/networkx/networkx/blob/main/networkx/algorithms/dag.py) along with

__all__ = [
    "descendants",
    "ancestors",
    "topological_sort",
    "lexicographical_topological_sort",
    "all_topological_sorts",
    "topological_generations",
    "is_directed_acyclic_graph",
    "is_aperiodic",
    "transitive_closure",
    "transitive_closure_dag",
    "transitive_reduction",
    "antichains",
    "dag_longest_path",
    "dag_longest_path_length",
    "dag_to_branching",
]

So, we initially placed this algorithm in the dag category.

[seunghwak]

Have you checked (with a small toy graph) what happens if the input graph has a cycle and do_expensive_check is set to false?

I guess in_degrees can have negative values and either we exits the loop pre-maturely or we fall into infinite loops.

We may accumulate the total aggregate frontier size and we may be able to detect pre-mature exit (if total < # vertices in the graph) or infinite loops (total > # vertices in the graph).

If there is no way to cheaply check this, we update documentation. Throws if the graph is symmetric or has cycles AND do_expensive_check is set to true. Otherwise, undefined behavior. If we can cheaply check this, we should check.