[Sparse] Add relabel API

[xiangyuzhi]

Description

Add a relabel python API on sparse.

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: 112a8d736a163086daba56adc6c36a3e8b3885e4

Build ID: 1

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 2139c08088b0aab5c78bacd6d56a405bf6086828

Build ID: 2

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: ec392fe5882648af032d43fcccd33c454d677a43

Build ID: 3

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 592b849186334fec54667f9d9c6e72b58737e009

Build ID: 4

Status: ❌ CI test failed in Stage [C++ CPU].

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[dgl-bot]

Commit ID: 3eaff48

Build ID: 5

Status: ❌ CI test failed in Stage [C++ CPU].

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[xiangyuzhi]

@dgl-bot

[dgl-bot]

Commit ID: 3eaff48

Build ID: 6

Status: ❌ CI test failed in Stage [C++ CPU].

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Full logs path: link

[dgl-bot]

Commit ID: 981cb36

Build ID: 7

Status: ❌ CI test failed in Stage [C++ CPU].

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