[DistGB] Integrate DistDGL with GraphBolt (sampling stage only)

[Rhett-Ying]

🔨Work Item

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Project tracker: https://github.com/orgs/dmlc/projects/2

Description

I have made a POC version in gb_distdgl branch on my forked repo. See more details in this doc.

Depending work items or issues

[Rhett-Ying]

@zheng-da @classicsong @frozenbugs @jermainewang FYI