[Sparse] Add compact operator

[xiangyuzhi]

Description

Sparse compact operator implementation.

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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Commit ID: 5a7558a

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Commit ID: d0d5032

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Commit ID: ea2cf72b780e76df040620b82c69f4c993951990

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Commit ID: 95e0cc8c23bf0eb72b26411fe77c093403a9a225

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Commit ID: a38eb3dd4ec1b4d7dd750ff7929fe1c89576a0a3

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Commit ID: 5082f5c8f9fd2e392418bf5fab6908e77c6a3260

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Commit ID: 9839faea74690c7b2707fa8b6b85605135ad26f6

Build ID: 7

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Commit ID: f09fb6222e25278e39c1433ea45afda294d4cf9e

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[dgl-bot]

Commit ID: f38374ec826044e206a572f54aeee2723e1c5237

Build ID: 9

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Commit ID: 146eae9b1fd952135e1db02385e301705f0d7bba

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Commit ID: 1da6a46f8083ee248b506eb7a691d3a792422426

Build ID: 11

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[dgl-bot]

Commit ID: 7c4724fac65c5c19bb1469645b9a22839a9c71a2

Build ID: 12

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Commit ID: 40585404b775a8210b80312ec947fc061bd6afb9

Build ID: 13

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Commit ID: 92486144df104b6e4efe6a38adf5706a50a52827

Build ID: 14

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[czkkkkkk]

LGTM with minor comments.

[dgl-bot]

Commit ID: 042615d051f22348ee24555fbfa8ac415e90529a

Build ID: 15

Status: ✅ CI test succeeded.

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Commit ID: e0275b746f667ebd0f35061202d21977386ec817

Build ID: 16

Status: ✅ CI test succeeded.

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