[GraphBolt] Labor example

[mfbalin]

Description

This example is meant to showcase the GPU feature cache and its interaction with the labor parameters layer_dependency and batch_dependency. Since this will replace the official labor example, performance is of utmost importance. I also use early stopping to show that you can get good accuracies with labor sampler and a custom GraphSAGE model.

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

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[mfbalin]

@frozenbugs I have moved the example under the PyG folder. Can you provide review for this PR? If DGL functionality is added later, it can be moved outside of the PyG examples folder.

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