[GraphBolt] fix preprocess issue for single ntype/etype graph

[Rhett-Ying]

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: 8995b6b7b462a6e88f90056a42038e2f3287f969

Build ID: 1

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 2a3e04dc761c39c3d0b0ebd0c2fbeff551d1d5ac

Build ID: 2

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

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Full logs path: link

[dgl-bot]

Commit ID: 5007d581533d70a9d45b8d0084cbb369f0554a04

Build ID: 3

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

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: 9caf2df9874f3e7f4350729131d5e668a77c2b84

Build ID: 4

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link