[GraphBolt][CUDA] Refactor `gb.unique_and_compact`, add `async_op`.

[mfbalin]

Description

Somehow, the PR ended up being this long because there are various branches.

I wonder if there is a way to generalize...

Checklist

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• [ ] The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• [ ] I've leverage the tools to beautify the python and c++ code.
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• [ ] All changes have test coverage
• [ ] Code is well-documented
• [ ] To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
• [ ] Related issue is referred in this PR
• [ ] If the PR is for a new model/paper, I've updated the example index here.

Changes

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