Nice work!
The version of implementation can reach 22+ BLUE score. However, my implementation have only 0.16+ BLUE score on test dataset. Comparing with your work, I found changing the concatenation torch.cat((Y_t, o_pre), dim=1) to torch.cat((o_pre, Y_t), dim=1) can only reach 0.16+ BLUE score.

B.W.T. chaging the concatenating order between dec_hidden and a_t in step function also