This reimplements the LASER encoder from:
```
Massively Multilingual Sentence Embeddings for Zero-Shot Cross-Lingual Transfer and Beyond
Mikel Artetxe, Holger Schwenk
https://arxiv.org/abs/1812.10464
```
and adds functionality to embed sentences with any Marian encoder, also different from LASER. Some early attempts to train a transformer model with Encoder-Decoder bottle-neck. This is quite early code, so some code-duplication is to be expected. Nevertheless, it's functional and I would like to have it in master as we will slowly put that into production in various places. I will make the code "nicer" as we go along.