Oh! Mr.@ptrblck ,  you remind me that In my nn.module model ---- net() , there is a class variable
self.previous_feature

it is used to store the extracted feature from the last training round.
BUT, it is like, updated every round, since it is always the current previous frame feature. Would it, say, just permanently stored in GPU? but it is in a updating mode~
OR! you remind me that :  the previous feature is on the computation graph, so even it is updated, the older one is still on the graph and not detached. that is the reason the updated feature still exists as a graph node. right?