[GraphBolt][CUDA] Add `gb.scatter` to support pinned tensors.

## 🔨Work Item

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

## Description

If we had such an operation, moving to different wouldn't be necessary below.

https://github.com/dmlc/dgl/blob/09e48552cbf8e1a60e8820d24753c3051623fdcb/examples/sampling/graphbolt/node_classification.py#L221-L223