Add GitHub Actions Workflow job to run NVTabular unit tests on GPU

[oliverholworthy]

Add GitHub Actions Workflow job to run NVTabular unit tests on GPU.

Motivation: To help us catch issues with GPU device functionality in NVTabular that relies on 'core' before a PR is merged

[karlhigley]

Historically we ran them on CPU only in order to avoid creating logjams on the GPU resources when the tests run in Core. This might be a reasonable and/or necessary thing to do, but we should tread carefully given the time it takes to run our test suites.

[github-actions[bot]]

Documentation preview

https://nvidia-merlin.github.io/core/review/pr-288