[Doc] Add documentation for DGLGraph.adj_sparse (#3096)

Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>

docs/source/api/python/dgl.DGLGraph.rst

@@ -143,6 +143,7 @@ Methods for getting the adjacency and the incidence matrix of the graph.
     :toctree: ../../generated/
 
     DGLGraph.adj
+    DGLGraph.adj_sparse
     DGLGraph.adjacency_matrix
     DGLGraph.inc
     DGLGraph.incidence_matrix

python/dgl/heterograph.py

@@ -3619,7 +3619,9 @@ class DGLHeteroGraph(object):
         --------
         >>> g = dgl.graph(([0, 1, 2], [1, 2, 3]))
         >>> g.adj_sparse('coo')
+        (tensor([0, 1, 2]), tensor([1, 2, 3]))
         >>> g.adj_sparse('csr')
+        (tensor([0, 1, 2, 3, 3]), tensor([1, 2, 3]), tensor([0, 1, 2]))
         """
         etid = self.get_etype_id(etype)
         if fmt == 'csc':