A few operators on graphs stored as CSR (apache#13290)

* edge_id op csr forward on CPU (#34)

* add node subgraph generator. (#35)

* create DGLSubgraph.

* fix.

* return old eids in node_subgraph.

* accelerate subgraph construction.

* Add neighborhood op (#37)

* add csr_neighborhood op

* update neighborhood sample

* Update csr_neighborhood_sample-inl.h

* Update csr_neighborhood_sample-inl.h

* Update csr_neighborhood_sample.cc

* add graph compact operator.

* fix a bug in dgl_subgraph.

* fix a bug in dgl_graph_compact.

* Update csr sample op (#39)

* add csr_neighborhood op

* update neighborhood sample

* Update csr_neighborhood_sample-inl.h

* Update csr_neighborhood_sample-inl.h

* Update csr_neighborhood_sample.cc

* Update csr_neighborhood_sample-inl.h

* Update csr_neighborhood_sample.cc

* Update csr_neighborhood_sample-inl.h

* remove space.

* move to dgl_graph to contrib.

* move code.

* move edge id.

* fix compilation error.

* add test for subgraph.

* cleanup.

* fix.

* fix.

* fix compile error.

* fix compile error.

* fix compile error.

* fix.

* add operator doc.

* remove graph_compact

* update doc.

* address comments.

* retrigger.

* address comments.

* retrigger

* fix a bug in test.

* retrigger

* add check_format

docs/api/python/ndarray/contrib.md

@@ -60,6 +60,7 @@ In the rest of this document, we list routines provided by the `ndarray.contrib`
     isnan
     index_copy
     getnnz
+    edge_id
 ```
 
 ## API Reference