GitLab

* subgraph copy from

* WIP

* cached members

* Change all usage of id tensor to the new Index object; remove set device in DGLGraph;

* subgraph merge API tested

* add dict type reduced msg test

* 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