Decoder embedding sharing in PyTorch Translate for denoising autoencoder (#386)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/386

Pull Request resolved: https://github.com/pytorch/translate/pull/266

This allows decoder embedding sharing for denoising autoencoder modules with different decoders (one for src decoding and one for tgt decoding)

Reviewed By: dpacgopinath

Differential Revision: D13133015

fbshipit-source-id: 3c98be639d705744ccf5ba3a8fd7d10ddc7aef4a