I’m not sure, if I completely understand your use case, but since you are using batch_first=True, the input is expected to have the shape [batch_size, seq_len, features], so 48 would be the sequence length? If so, then input_lengths would be wrong since it’s returning a tensor containing 76 for each sample.  Once this is fixed, you would have to check the lstm_out, as it would also be a PackedSequence, so I guess you might want to access the return value via lstm_out.data.