[GraphBolt] gb.DataLoader can simply be a datapipe.

[mfbalin]

Description

There is no need to derive from DataLoader as we are not using any of its functionalities.

I also remove some datapipe modification only meant for num_workers>0 to a if num_workers > 0: branch.

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: dadb638

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Commit ID: f703e552a23097b70bc47cd2f7f6152da922f45d

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Commit ID: 6edc601

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Commit ID: cabf8b2

Build ID: 4

Status: ✅ CI test succeeded.

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[Rhett-Ying]

why the expected results are changed?

[mfbalin]

I guess torch.utils.data.DataLoader does things that we don't know about if simply inheriting from it has side effects.

[mfbalin]

The code is here, it is quite long so we need good justification to keep deriving from it. https://github.com/pytorch/pytorch/blob/main/torch/utils/data/dataloader.py

[frozenbugs]

This is weird, it seems that the seed is respected in a different way.

[mfbalin]

@frozenbugs this is a small PR that can potentially affect performance slightly (The less code we run, the potential to become faster.). Do you want to take a quick look as well?

[dgl-bot]

Commit ID: f140011969eece3cecf589ddb7437f4b0d9764fb

Build ID: 5

Status: ⚪️ CI test cancelled due to overrun.

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Commit ID: 19e507c765b22aa42eced1774347d3859484ef57

Build ID: 6

Status: ✅ CI test succeeded.

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[frozenbugs]

@dgl-bot