* subgraph
* more test cases
* WIP
* new FrameRef and test
* separate nx init code
* WIP
* subgraph code and test
* line graph code and test
* adding new test for adding new features on line graphs
* no backtracking line graph
* fix inplace relabel
* fix edge list order problem in cached graph.
* minor fix
* fix bug in edge iter
* SPMV works
* gcn spmv on CPU
* change gcn style
* fix cached graph performance; fixed gcn dataset bug
* reorg dir
* non-batch spmv; partial update problem with shape change
* fix reorder problem; finish gcn-batch impl
* pop API
* GPU context