[GraphBolt][CUDA] Investigate UVA thread count default value.

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

## Description

https://github.com/dmlc/dgl/blob/fe78093fb74719e39e4bba372d44822a4be8c568/python/dgl/graphbolt/dataloader.py#L172

Investigate whether this default is good for different scenarios and systems. If it is not, we should set the default to a higher value to be safe.

@TristonC