[GraphBolt] change default value of persistent_workers to False

[Rhett-Ying]

Description

to be consistent with default value in torch.utils.data.DataLoader which is False in default.

I don't specify persistent_workers=True explicitly for our examples as num_workers=0 is used in default. And I'd like to see if such change triggers regression.

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: a40f283

Build ID: 1

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 9e836de

Build ID: 2

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

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[dgl-bot]

Commit ID: 9ea15da7afab433ce558f93bcda16308a0e11903

Build ID: 3

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 4b76b8a79c0d75bcaadb3e8d068b54e7c6aaa004

Build ID: 4

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: d5c06806816016712fe05d5f365a29c0c7ee3ab4

Build ID: 5

Status: ✅ CI test succeeded.

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