[GraphBolt][CUDA] Eliminate synchronization in link prediction preprocessing.

[mfbalin]

🔨Work Item

IMPORTANT:

• This template is only for dev team to track project progress. For feature request or bug report, please use the corresponding issue templates.
• DO NOT create a new work item if the purpose is to fix an existing issue or feature request. We will directly use the issue in the project tracker.

Project tracker: https://github.com/orgs/dmlc/projects/2

Description

SubgraphSampler.preprocess function has a unique_and_compact, which requires us to step in to hide the synchronization.

dgl/python/dgl/graphbolt/subgraph_sampler.py

Line 166 in 8772c02

unique_seeds, compacted = unique_and_compact(nodes)

dgl/python/dgl/graphbolt/subgraph_sampler.py

Line 215 in 8772c02

unique_seeds, compacted = unique_and_compact(nodes)