'Builds the sentence decoders.
Inputs:
self.decode_pre_emb
self.decode_post_emb
self.decode_pre_ids
self.decode_post_ids
self.decode_pre_mask
self.decode_post_mask
self.thought_vectors
Outputs:
self.target_cross_entropy_losses
self.target_cross_entropy_loss_weights'

def build_decoders(self):

    if (self.mode != 'encode'):
        self._build_decoder('decoder_pre', self.decode_pre_emb, self.decode_pre_ids, self.decode_pre_mask, self.thought_vectors, False)
        self._build_decoder('decoder_post', self.decode_post_emb, self.decode_post_ids, self.decode_post_mask, self.thought_vectors, True)
