[Model] GraphSAGE recommendation with custom distributed sampler

[BarclayII]

Description

GraphSAGE + custom distributed sampler on MovieLens dataset, both for rating prediction and link predictions.

This also includes a custom distributed sampler which sends auxiliary data along with NodeFlows.

As a result, two C APIs SerializeSubgraph and DeserializeSubgraph are added, which converts between C NodeFlow Handles and Python bytearray's.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [Model], [Doc], [Feature]])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• [?] Code is well-documented
• To the my best knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change

[jermainewang]

@yzh119 Is it possible to implement GraphSAGEConv in nn module first? This application could be a good test for the module.