If your remove batch_first=True it’s of course batch_first=False by default. In this case you would need to change out = self.fc(out[:, -1, :]) to out = self.fc(out[-1])
I don’t what you’re trying to learn and how your data looks like, but x = x.reshape(-1, INPUT_SIZE, MAX_STRING_SIZE) looks a but suspicious. Firstly, why do you need to infer the batch size (I assume batch_first=True and the first dimension is for the batch). And secondly, input_size is expected to be in the 3rd dimension of the input tensors for a LSTM/GRU.
I’m not sure about your reshape() in the return line either. If you’re not careful reshape() and view() can quickly mess up your data.