[Example] Fix error in gcn train due to missing argument (#3040)

* Fix example

* Remove register_data_args and revert  to dataset
Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>

examples/mxnet/gcn/train.py

 """Training GCN model on citation graphs."""
 import argparse, time
 import numpy as np
-import networkx as nx
 import mxnet as mx
 from mxnet import gluon
 
 import dgl
-from dgl.data import register_data_args
 from dgl.data import CoraGraphDataset, CiteseerGraphDataset, PubmedGraphDataset
 
 from gcn import GCN
@@ -112,7 +110,8 @@ def main(args):
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description='GCN')
-    register_data_args(parser)
+    parser.add_argument("--dataset", type=str, default="cora",
+                        help="Dataset name ('cora', 'citeseer', 'pubmed').")
     parser.add_argument("--dropout", type=float, default=0.5,
             help="dropout probability")
     parser.add_argument("--gpu", type=int, default=-1,

examples/pytorch/gcn/train.py

 import argparse
 import time
 import numpy as np
-import networkx as nx
 import torch
-import torch.nn as nn
 import torch.nn.functional as F
 import dgl
-from dgl.data import register_data_args
 from dgl.data import CoraGraphDataset, CiteseerGraphDataset, PubmedGraphDataset
 
 from gcn import GCN
@@ -123,7 +120,8 @@ def main(args):
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description='GCN')
-    register_data_args(parser)
+    parser.add_argument("--dataset", type=str, default="cora",
+                        help="Dataset name ('cora', 'citeseer', 'pubmed').")
     parser.add_argument("--dropout", type=float, default=0.5,
                         help="dropout probability")
     parser.add_argument("--gpu", type=int, default=-1,

examples/tensorflow/gcn/train.py

 import argparse
 import time
 import numpy as np
-import networkx as nx
 import tensorflow as tf
 import dgl
-from dgl.data import register_data_args
 from dgl.data import CoraGraphDataset, CiteseerGraphDataset, PubmedGraphDataset
 from gcn import GCN
 
@@ -116,7 +114,8 @@ def main(args):
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description='GCN')
-    register_data_args(parser)
+    parser.add_argument("--dataset", type=str, default="cora",
+                        help="Dataset name ('cora', 'citeseer', 'pubmed').")
     parser.add_argument("--dropout", type=float, default=0.5,
                         help="dropout probability")
     parser.add_argument("--gpu", type=int, default=-1,