GitLab

* add edge sampler.

* add test and run.

* add negative sampling.

* remap the edge subgraph vid.

* negative graph excludes edges of positive edgs.

* remove print.

* avoid sampling NodeFlow when expand_factor or num_hops is 0.

* fix a bug when excluding nodes in negative graph.

* support multigraph.

* exclude positive edges.

* fix memory leak.

* return subgraph object directly.

* fix many problems.

* add comments.

* address comments