GitLab

* WIP: api

* dgl.sampling, dgl.data

* dgl.sampling; dgl.dataloading

* sampling packages

* convert

* subgraph

* deprecate

* subgraph APIs

* All docstrings for convert/subgraph/transform

* almost all funcs under dgl namespace

* WIP: DGLGraph

* done graph query

* message passing functions

* lint

* fix merge error

* fix test

* lint

* fix
Co-authored-by: Quan Gan <coin2028@hotmail.com>