[GraphBolt] Refactor datapipes folder and add to_graph.

[mfbalin]

Description

Organizing datapipes folder and adding to_graph for visualizing datapipes. Moved datapipes out of internal because torchdata is unusable together with GraphBolt. Thus, if users need datapipes, they need to be able to import and use them.

to_graph is extremely useful for debugging to see what is in the created data pipeline.

Can be used like: to_graph(datapipe).render("gb.DataLoader")

Like so, this is low resolution, can be made high resolution as well:

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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