Change default word_size=64 in the extension for CUDA.jl

[ytdHuang]

As we discussed in Zulip, we change the default word_size to 64 for the function cu in CUDA extension.

I also add the brief example (in README and documentation) to the runtest for CUDA extension. But since we don't have access to JuliaGPU pipeline yet, so I skipped the CI.