[GraphBolt][CUDA] Add FeatureCache::IndexSelect.

[mfbalin]

Description

This op is needed for zero copy access from the GPU. GPU reading tests will be included in a follow-up PR when I make use of this added functionality.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• I've leverage the tools to beautify the python and c++ code.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• All changes have test coverage
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
• Related issue is referred in this PR
• If the PR is for a new model/paper, I've updated the example index here.

Changes

[dgl-bot]

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[frozenbugs]

Index select and pin memory are not related, could be 2 small prs.

[mfbalin]

Index select and pin memory are not related, could be 2 small prs.

Indeed.