[GraphBolt][CUDA] Make fetching indices optional for NS.

[mfbalin]

Description

When indices is pinned, its fetch takes long. We want to actually overlap its fetch operation. We already have a overlap_graph_fetch option that fetches the full InSubgraph. However, this is wasteful for NeighborSampling as it only requires fetching the indices for the sampled edges. Thus, we make it optional so that it can be fetched in another stage during sampling in an overlapped way.

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]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: 9c09380

Build ID: 1

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 93a519e

Build ID: 2

Status: ✅ CI test succeeded.

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[frozenbugs]

I didn't catch it in previous review, but layer is not a good name for boolean.