OpenNMT
Also an open-source sequence-to-sequence toolkit but with a simpler interface and broader community.
Tensor2Tensor
Google’s library for training deep learning models with a focus on transformers and NLP tasks.
Hugging Face Transformers
Offers a large collection of pretrained transformer models with easy-to-use APIs for NLP tasks.
Marian NMT
Specialized in neural machine translation with efficient training and inference.