[GraphBolt][CUDA] Specialize non-weighted neighbor sampling impl

[mfbalin]

Fixes #7173. Neighbor Sampling kernel is sped up by 4-5x compared to old, non-specialized version.

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: cd4b0e1

Build ID: 1

Status: ❌ CI test failed in Stage [GPU Build].

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[mfbalin]

@TristonC The CI has the following error message:

/home/ubuntu/jenkins/workspace/dgl_PR-7215/third_party/cccl/libcudacxx/include/cuda/std/detail/libcxx/include/support/atomic/atomic_cuda.h:12:4: error: #error "CUDA atomics are only supported for sm_60 and up on *nix and sm_70 and up on Windows."
   12 | #  error "CUDA atomics are only supported for sm_60 and up on *nix and sm_70 and up on Windows."

I am going to work around it.

[dgl-bot]

Commit ID: cc4f6b2

Build ID: 2

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 68c0da3

Build ID: 3

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: adb35df

Build ID: 4

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 69575c3

Build ID: 5

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 8811f46691dc68d496e777217070f33ef8e1d5fa

Build ID: 6

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: c093e65

Build ID: 7

Status: ✅ CI test succeeded.

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