Lost_Wanderer:

I thought that nn.GRU and nn.LSTM differed only in their internal implementation and that their inputs and outptus were the same.


No, that’s not the case, as their states are different, which also seems to case the issues.
You can see the expected inputs in the docs for nn.GRU and nn.LSTM.
E.g. the first error is raised, since the LSTM returns one output tensor and two states.