[GraphBolt][CUDA] Fix link prediction early-stop.

[mfbalin]

Description

Fixes #7082. When feature overlap is enabled, the dataloader has buffers for the future feature fetches. When we reuse the same dataloader after stopping early, it tries to pickle the objects in the buffers, then tries the pickle CUDAEvent stored in the object stored in the buffer. Creating a new dataloader after stopping early avoids this issue.

More details and a potential alternative fix here: pytorch/data#1161

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.
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• 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: ba4ee3b

Build ID: 1

Status: ✅ CI test succeeded.

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