[Feature] GPU traversal

🚀 Feature

GPU traversal (dfs/bfs/topological/...)

Motivation

Currently we only implement single thread traversal on CPU, it's not efficient and the frontiers cannot be generated on-the-fly with message passing.

Pitch

This feature is important for users who are dealing with graphs with a large number of nodes(edges), e.g. @nforest is working on program analysis where dgl traversal becomes their bottleneck.

There has been many literatures working on Graph Traversal on GPU, to name a few:

1. Gunrock: GPU Graph Analytics, TOPC
2. GPU-based Graph Traversal on Compressed Graphs, SIGMOD
3. ...

we can borrow the ideas from these papers and make a traversal on GPU that could generate frontiers on-the-fly with the execution of message function and reduce function. As the design of our built-in function is based on (a minimized) gunrock, I suppose it would not be too hard to implement a gunrock-like traversal algorithm.