[Sparse] Compact C++ API

[xiangyuzhi]

Description

Add C++ API for compact operation.

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: d9c4173

Build ID: 1

Status: ❌ CI test failed in Stage [Lint Check].

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Full logs path: link

[dgl-bot]

Commit ID: aea7281

Build ID: 2

Status: ❌ CI test failed in Stage [CPU Build (Win64)].

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Full logs path: link

[dgl-bot]

Commit ID: c091a704ab98e4e5d6e45056f7968f71e23e0bf2

Build ID: 3

Status: ✅ CI test succeeded.

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Full logs path: link

[dgl-bot]

Commit ID: 3212a3d01abbdc9cebbf1da15042fe59977aa7e7

Build ID: 4

Status: ❌ CI test failed in Stage [Lint Check].

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Full logs path: link

[dgl-bot]

Commit ID: 95ecc18d82f6040b2c7a3ea73b0076d748fea0ac

Build ID: 5

Status: ✅ CI test succeeded.

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Full logs path: link

[dgl-bot]

Commit ID: 3bcf73fc638b7159cc836a352cf1c05eec2ea87e

Build ID: 6

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link