[Example] Remove duplicated edges in RRN Sudoku example (#1946)

* PPIDataset

* Revert "PPIDataset"

This reverts commit 264bd0c960cfa698a7bb946dad132bf52c2d0c8a.

* remove duplicate edges

* Update README.md

examples/pytorch/rrn/README.md

 # Recurrent Relational Network (RRN)
 
 * Paper link: https://arxiv.org/abs/1711.08028
-* Author's code repo: https://github.com/rasmusbergpalm/recurrent-relational-networks.git
+* Author's code repo: https://github.com/rasmusbergpalm/recurrent-relational-networks
 
 ## Dependencies
 
 * PyTorch 1.0+
-* DGL 0.3+
+* DGL 0.5+
 
 ## Codes
 

examples/pytorch/rrn/sudoku_data.py

@@ -20,25 +20,24 @@ def _basic_sudoku_graph():
              [54, 55, 56, 63, 64, 65, 72, 73, 74],
              [57, 58, 59, 66, 67, 68, 75, 76, 77],
              [60, 61, 62, 69, 70, 71, 78, 79, 80]]
-    g = dgl.DGLGraph()
-    g.add_nodes(81)
+    edges = set()
     for i in range(81):
         row, col = i // 9, i % 9
         # same row and col
         row_src = row * 9
         col_src = col
         for _ in range(9):
-            if row_src != i:
-                g.add_edges(row_src, i)
-            if col_src != i:
-                g.add_edges(col_src, i)
+            edges.add((row_src, i))
+            edges.add((col_src, i))
             row_src += 1
             col_src += 9
         # same grid
         grid_row, grid_col = row // 3, col // 3
         for n in grids[grid_row*3 + grid_col]:
             if n != i:
-                g.add_edges(n, i)
+                edges.add((n, i))
+    edges = list(edges)
+    g = dgl.graph(edges)
     return g
 
 
@@ -83,6 +82,7 @@ def _get_sudoku_dataset(segment='train'):
         return encoded
 
     data = encode(data)
+    print(f'Number of puzzles in {segment} set : {len(data)}')
 
     return data