Your right, since the output of the encoder is just a sentence embedding, you can use an existing encoding sentence embedding and use only the decoder part.
A couple of days ago, there was this post 3 about the implementation in a paper, where the embedding came from CNN and was fed into an LSTM decoder. In this paper – as far as I could understand – the authors set the intial hidden state h_0 and zero-ed the initial cell state c_0. I would simply try this approach, as well as:

use the embedding to set h_0 and c_0

use a nn.GRU which doesn’t have cell state 


and just see how this effects the results.