[examples] create dataloader before train epoch (#6636)

examples/sampling/graphbolt/rgcn/hetero_rgcn.py

@@ -537,14 +537,6 @@ def run(
     print("start to run...")
     category = "paper"
 
-    # Typically, the best Validation performance is obtained after
-    # the 1st or 2nd epoch. This is why the max epoch is set to 3.
-    for epoch in range(3):
-        num_train = len(train_set)
-        model.train()
-
-        total_loss = 0
-
     data_loader = create_dataloader(
         name,
         g,
@@ -556,6 +548,15 @@ def run(
         shuffle=True,
         num_workers=num_workers,
     )
+
+    # Typically, the best Validation performance is obtained after
+    # the 1st or 2nd epoch. This is why the max epoch is set to 3.
+    for epoch in range(3):
+        num_train = len(train_set)
+        model.train()
+
+        total_loss = 0
+
         for data in tqdm(data_loader, desc=f"Training~Epoch {epoch:02d}"):
             # Convert MiniBatch to DGL Blocks.
             blocks = [block.to(device) for block in data.blocks]