GitLab

* add neighbor sampler.

* fix sampler.

* handle non-existing nodes differently in MapParentIdToSubgraphId.

* fix sampling tests.

* parallelize sampling.

* add more tests.

* fix a bug in subgraph loader.

* get number of sampled nodes directly.

* load mxnet CSR to DGLGraph directly.

* create graph from mxnet csr array directly.

* use subgraph loader in SSE.

* fix a bug in sampling.

* output framework tensor.

* fix.

* remove unnecessary code.

* fix tests.

* add SSE README.

* move to contrib.

* address comments.

* add tests.