[Feature] Add reverse edges in CitationGraphDataset (#2588)

* [Feature] Add reverse edges in CitationGraphDataset

* fix bug

* fix bug

* fix bug

* Update python/dgl/data/citation_graph.py
Co-authored-by: Xiangkun Hu <huxk_hit@qq.com>

* fix notes

* fix notes

* fix bugs

* solve requested changes

* fix bug

* fix bug
Co-authored-by: Xiangkun Hu <huxk_hit@qq.com>

python/dgl/data/citation_graph.py

@@ -43,7 +43,9 @@ class CitationGraphDataset(DGLBuiltinDataset):
     force_reload : bool
         Whether to reload the dataset. Default: False
     verbose: bool
-      Whether to print out progress information. Default: True.
+        Whether to print out progress information. Default: True.
+    reverse_edge: bool
+        Whether to add reverse edges in graph. Default: True.
     """
     _urls = {
         'cora_v2' : 'dataset/cora_v2.zip',
@@ -51,7 +53,7 @@ class CitationGraphDataset(DGLBuiltinDataset):
         'pubmed' : 'dataset/pubmed.zip',
     }
 
-    def __init__(self, name, raw_dir=None, force_reload=False, verbose=True):
+    def __init__(self, name, raw_dir=None, force_reload=False, verbose=True, reverse_edge=True):
         assert name.lower() in ['cora', 'citeseer', 'pubmed']
 
         # Previously we use the pre-processing in pygcn (https://github.com/tkipf/pygcn)
@@ -60,6 +62,8 @@ class CitationGraphDataset(DGLBuiltinDataset):
             name = 'cora_v2'
 
         url = _get_dgl_url(self._urls[name])
+        self._reverse_edge = reverse_edge
+
         super(CitationGraphDataset, self).__init__(name,
                                                    url=url,
                                                    raw_dir=raw_dir,
@@ -104,7 +108,11 @@ class CitationGraphDataset(DGLBuiltinDataset):
 
         features = sp.vstack((allx, tx)).tolil()
         features[test_idx_reorder, :] = features[test_idx_range, :]
-        graph = nx.DiGraph(nx.from_dict_of_lists(graph))
+
+        if self.reverse_edge:
+            graph = nx.DiGraph(nx.from_dict_of_lists(graph))
+        else:
+            graph = nx.Graph(nx.from_dict_of_lists(graph))
 
         onehot_labels = np.vstack((ally, ty))
         onehot_labels[test_idx_reorder, :] = onehot_labels[test_idx_range, :]
@@ -254,6 +262,11 @@ class CitationGraphDataset(DGLBuiltinDataset):
         deprecate_property('dataset.feat', 'g.ndata[\'feat\']')
         return self._g.ndata['feat']
 
+    @property
+    def reverse_edge(self):
+        return self._reverse_edge
+    
+
 def _preprocess_features(features):
     """Row-normalize feature matrix and convert to tuple representation"""
     rowsum = np.asarray(features.sum(1))
@@ -343,6 +356,8 @@ class CoraGraphDataset(CitationGraphDataset):
         Whether to reload the dataset. Default: False
     verbose: bool
         Whether to print out progress information. Default: True.
+    reverse_edge: bool
+        Whether to add reverse edges in graph. Default: True.
 
     Attributes
     ----------
@@ -385,10 +400,10 @@ class CoraGraphDataset(CitationGraphDataset):
     >>> # Train, Validation and Test
 
     """
-    def __init__(self, raw_dir=None, force_reload=False, verbose=True):
+    def __init__(self, raw_dir=None, force_reload=False, verbose=True, reverse_edge=True):
         name = 'cora'
 
-        super(CoraGraphDataset, self).__init__(name, raw_dir, force_reload, verbose)
+        super(CoraGraphDataset, self).__init__(name, raw_dir, force_reload, verbose, reverse_edge)
 
     def __getitem__(self, idx):
         r"""Gets the graph object
@@ -483,6 +498,8 @@ class CiteseerGraphDataset(CitationGraphDataset):
         Whether to reload the dataset. Default: False
     verbose: bool
         Whether to print out progress information. Default: True.
+    reverse_edge: bool
+        Whether to add reverse edges in graph. Default: True.
 
     Attributes
     ----------
@@ -528,10 +545,10 @@ class CiteseerGraphDataset(CitationGraphDataset):
     >>> # Train, Validation and Test
 
     """
-    def __init__(self, raw_dir=None, force_reload=False, verbose=True):
+    def __init__(self, raw_dir=None, force_reload=False, verbose=True, reverse_edge=True):
         name = 'citeseer'
 
-        super(CiteseerGraphDataset, self).__init__(name, raw_dir, force_reload, verbose)
+        super(CiteseerGraphDataset, self).__init__(name, raw_dir, force_reload, verbose, reverse_edge)
 
     def __getitem__(self, idx):
         r"""Gets the graph object
@@ -626,6 +643,8 @@ class PubmedGraphDataset(CitationGraphDataset):
         Whether to reload the dataset. Default: False
     verbose: bool
         Whether to print out progress information. Default: True.
+    reverse_edge: bool
+        Whether to add reverse edges in graph. Default: True.
 
     Attributes
     ----------
@@ -668,10 +687,10 @@ class PubmedGraphDataset(CitationGraphDataset):
     >>> # Train, Validation and Test
 
     """
-    def __init__(self, raw_dir=None, force_reload=False, verbose=True):
+    def __init__(self, raw_dir=None, force_reload=False, verbose=True, reverse_edge=True):
         name = 'pubmed'
 
-        super(PubmedGraphDataset, self).__init__(name, raw_dir, force_reload, verbose)
+        super(PubmedGraphDataset, self).__init__(name, raw_dir, force_reload, verbose, reverse_edge)
 
     def __getitem__(self, idx):
         r"""Gets the graph object
@@ -699,7 +718,7 @@ class PubmedGraphDataset(CitationGraphDataset):
         r"""The number of graphs in the dataset."""
         return super(PubmedGraphDataset, self).__len__()
 
-def load_cora(raw_dir=None, force_reload=False, verbose=True):
+def load_cora(raw_dir=None, force_reload=False, verbose=True, reverse_edge=True):
     """Get CoraGraphDataset
 
     Parameters
@@ -711,15 +730,17 @@ def load_cora(raw_dir=None, force_reload=False, verbose=True):
         Whether to reload the dataset. Default: False
     verbose: bool
     Whether to print out progress information. Default: True.
+    reverse_edge: bool
+        Whether to add reverse edges in graph. Default: True.
 
     Return
     -------
     CoraGraphDataset
     """
-    data = CoraGraphDataset(raw_dir, force_reload, verbose)
+    data = CoraGraphDataset(raw_dir, force_reload, verbose, reverse_edge)
     return data
 
-def load_citeseer(raw_dir=None, force_reload=False, verbose=True):
+def load_citeseer(raw_dir=None, force_reload=False, verbose=True, reverse_edge=True):
     """Get CiteseerGraphDataset
 
     Parameters
@@ -731,15 +752,17 @@ def load_citeseer(raw_dir=None, force_reload=False, verbose=True):
         Whether to reload the dataset. Default: False
     verbose: bool
     Whether to print out progress information. Default: True.
+    reverse_edge: bool
+        Whether to add reverse edges in graph. Default: True.
 
     Return
     -------
     CiteseerGraphDataset
     """
-    data = CiteseerGraphDataset(raw_dir, force_reload, verbose)
+    data = CiteseerGraphDataset(raw_dir, force_reload, verbose, reverse_edge)
     return data
 
-def load_pubmed(raw_dir=None, force_reload=False, verbose=True):
+def load_pubmed(raw_dir=None, force_reload=False, verbose=True, reverse_edge=True):
     """Get PubmedGraphDataset
 
     Parameters
@@ -751,12 +774,14 @@ def load_pubmed(raw_dir=None, force_reload=False, verbose=True):
             Whether to reload the dataset. Default: False
         verbose: bool
         Whether to print out progress information. Default: True.
+    reverse_edge: bool
+        Whether to add reverse edges in graph. Default: True.
 
     Return
     -------
     PubmedGraphDataset
     """
-    data = PubmedGraphDataset(raw_dir, force_reload, verbose)
+    data = PubmedGraphDataset(raw_dir, force_reload, verbose, reverse_edge)
     return data
 
 class CoraBinary(DGLBuiltinDataset):