[doc] fix undefined variable in example

docs/source/guide/minibatch-node.rst

@@ -45,6 +45,7 @@ putting the list of generated MFGs onto GPU.
     device = torch.device('cuda' if torch.cuda.is_available() else 'cpu')
     dataset = gb.BuiltinDataset("ogbn-arxiv").load()
     g = dataset.graph
+    feature = dataset.feature
     train_set = dataset.tasks[0].train_set
     datapipe = gb.ItemSampler(train_set, batch_size=1024, shuffle=True)
     datapipe = datapipe.sample_neighbor(g, [10, 10]) # 2 layers.
@@ -207,6 +208,7 @@ of node types to node IDs.
     device = torch.device('cuda' if torch.cuda.is_available() else 'cpu')
     dataset = gb.BuiltinDataset("ogbn-mag").load()
     g = dataset.graph
+    feature = dataset.feature
     train_set = dataset.tasks[0].train_set
     datapipe = gb.ItemSampler(train_set, batch_size=1024, shuffle=True)
     datapipe = datapipe.sample_neighbor(g, [10, 10]) # 2 layers.