In a source autoencoder, the source-to-target model P (y|x; − → θ ) serves as an encoder to transform the observed source sentence x into a latent target sentence y (highlighted in grey), from which the target-to-source model P (x |y; ← − θ ) reconstructs a copy of the observed source sentence x from the latent target sentence.
monolingual corpora can be combined with parallel corpora to train bidirectional NMT models in a semi-supervised setting.