attribute generation for pretrain_OAG

[shifengzhao]

Thanks again for this awesome repo. It helps me a lot. But I have a question for attribute generation about pretrain_OAG.
When we generate i-th attribute, i+1 node should not be accepted. However, in pretrain_OAG, all source node attributes are not masked, so if there exists the situation i-th node can accept i+1 node?
for example:
i-th node is a "paper" node, i+1-th node is another "paper" node which cites the i-the "paper" node.