[GraphBolt][CUDA] Make _convert_to_sampled_subgraph lighter.

[mfbalin]

Incorporate rest of the _convert_to_sampled_subgraph into SampleNeighbors.

Description

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: d99e8ff

Build ID: 2

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 543e4a436c6b6219ce64b414e22d93f30ac654d5

Build ID: 1

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: d99e8ff

Build ID: 3

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 4e9fb6ad0610c9cea01a852beed2a42b5e02fb02

Build ID: 4

Status: ⚪️ CI test cancelled due to overrun.

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[mfbalin]

@frozenbugs looks like lintrunner is broken now.

[dgl-bot]

Commit ID: e7016df

Build ID: 5

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 3e9e31ebf1c2ba9713f715dcc2d89a99ad7aafeb

Build ID: 6

Status: ✅ CI test succeeded.

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