[GraphBolt][CUDA] Enable GPU sampling in examples

[mfbalin]

Description

Making modifications to enable GPU sampling in the graphbolt examples. All modified examples seem to just work, haven't tested the multi-GPU lightning scenario yet (num_gpus > 1). I would appreciate it if you could help see if they all work and match the CPU sampling results.

Depends on #6957.

Google colab demo here: https://colab.research.google.com/drive/1AMnK9YlEP5dSWiz1Kg6V6DdVjsiSeN75?usp=sharing

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