* rgcn tutorial * data processing for rgcn * many fix * requirements for rgcn tutorial * fix comments * description of rgcn dataset * author * bug fix * move all dataset to s3
* 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.
Lingfan Yu