[GraphBolt] Make unique_and_compact batched.

[mfbalin]

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Project tracker: DGL Project Tracker

Description

Calling unique_and_compact for each etype is too expensive, especially on the CUDA backend. We need to make a batched version of it so that it can handle all etypes in a single call inside C++. #7239 adds the API. The algorithm is not really batched though.

@frozenbugs @peizhou001 @Rhett-Ying