[Feature] K-hop subgraph sampling with support for heterogeneous directed graphs

[ali6947]

Description

Here is an API for sampling a k-hop subgraph around a node in a heterogenous directed graph. Prior to this, there was an API for only in-subgraph and out-neighborhood but we add support of for traversing edges in both directions when sampling the subgraph. This is useful for extracting heterogenous subgraphs around a node for training GNNs

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

Changes

• API Implementation of the function khop_subgraph
• Tests added
• Documentation provided

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@ali6947 Thanks for the contribution. We will discuss how to proceed for this PR.

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Commit ID: 300293475b108f8da25046e88d718696f52a107f

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