[Sparse] APPNP example (#4919)

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Co-authored-by: Ubuntu <ubuntu@ip-172-31-36-188.ap-northeast-1.compute.internal>

examples/sparse/appnp.py

+"""
+[Predict then Propagate: Graph Neural Networks meet Personalized PageRank]
+(https://arxiv.org/abs/1810.05997)
+"""
+
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+from dgl.data import CoraGraphDataset
+from dgl.mock_sparse import create_from_coo, diag, identity
+from torch.optim import Adam
+
+
+class APPNP(nn.Module):
+    def __init__(
+        self,
+        in_size,
+        out_size,
+        hidden_size=64,
+        dropout=0.1,
+        num_hops=10,
+        alpha=0.1,
+    ):
+        super().__init__()
+
+        self.f_theta = nn.Sequential(
+            nn.Dropout(dropout),
+            nn.Linear(in_size, hidden_size),
+            nn.ReLU(),
+            nn.Dropout(dropout),
+            nn.Linear(hidden_size, out_size),
+        )
+        self.num_hops = num_hops
+        self.A_dropout = nn.Dropout(dropout)
+        self.alpha = alpha
+
+    def forward(self, A_hat, X):
+        A_val_0 = A_hat.val
+        Z_0 = Z = self.f_theta(X)
+        for _ in range(self.num_hops):
+            A_hat.val = self.A_dropout(A_val_0)
+            Z = (1 - self.alpha) * A_hat @ Z + self.alpha * Z_0
+        # Reset A_hat.val to avoid value corruption.
+        A_hat.val = A_val_0
+        return Z
+
+
+def evaluate(g, pred):
+    label = g.ndata["label"]
+    val_mask = g.ndata["val_mask"]
+    test_mask = g.ndata["test_mask"]
+
+    # Compute accuracy on validation/test set.
+    val_acc = (pred[val_mask] == label[val_mask]).float().mean()
+    test_acc = (pred[test_mask] == label[test_mask]).float().mean()
+    return val_acc, test_acc
+
+
+def train(g, A_hat, X, model):
+    label = g.ndata["label"]
+    train_mask = g.ndata["train_mask"]
+    optimizer = Adam(model.parameters(), lr=1e-2, weight_decay=5e-4)
+
+    for epoch in range(50):
+        # Forward.
+        model.train()
+        logits = model(A_hat, X)
+
+        # Compute loss with nodes in training set.
+        loss = F.cross_entropy(logits[train_mask], label[train_mask])
+
+        # Backward.
+        optimizer.zero_grad()
+        loss.backward()
+        optimizer.step()
+
+        # Compute prediction.
+        model.eval()
+        logits = model(A_hat, X)
+        pred = logits.argmax(dim=1)
+
+        # Evaluate the prediction.
+        val_acc, test_acc = evaluate(g, pred)
+        print(
+            f"In epoch {epoch}, loss: {loss:.3f}, val acc: {val_acc:.3f}, test"
+            f" acc: {test_acc:.3f}"
+        )
+
+
+if __name__ == "__main__":
+    # If CUDA is available, use GPU to accelerate the training, use CPU
+    # otherwise.
+    dev = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")
+
+    # Load graph from the existing dataset.
+    dataset = CoraGraphDataset()
+    g = dataset[0].to(dev)
+
+    # Create the sparse adjacency matrix A.
+    src, dst = g.edges()
+    N = g.num_nodes()
+    A = create_from_coo(dst, src, shape=(N, N))
+
+    # Calculate the symmetrically normalized adjacency matrix.
+    I = identity(A.shape, device=dev)
+    A_hat = A + I
+    D_hat = diag(A_hat.sum(dim=1)) ** -0.5
+    A_hat = D_hat @ A_hat @ D_hat
+
+    # Create APPNP model.
+    X = g.ndata["feat"]
+    in_size = X.shape[1]
+    out_size = dataset.num_classes
+    model = APPNP(in_size, out_size).to(dev)
+
+    # Kick off training.
+    train(g, A_hat, X, model)