【GraphBolt】【HeteroGraph】HeteroGraph can not generate batch

## 🐛 Bug
When I was using GraphBolt for a heterogeneous graph link prediction task, errors frequently occurred during batch generation. I created a dataset called HGBl-amazon, which includes one type of node: product, and two types of edges: Product-0-Product and Product-1-Product. I constructed a link prediction task and stored edge information in the train_set, val_set and test_set like GraphBolt examples. However, I always encountered errors while iterating through the dataloader.

code:
self.model.train()
loss_all = 0.0
for i, data in enumerate(self.train_dataloader): # this line always raise error.