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New features:
* Add new tasks for Almond: almond_dialogue_nlu, almond_dialogue_nlg, almond_dialogue_policy
* Added a new encoder, "Coattention", which encodes the context and question separately, then
coattends and applies a BiLSTM layer.
* For Coattention and Identity encoder, it is now possible to specify the context and question
embeddings separately.
* Embeddings in context, question and answer can now be untied, by suffixing the name with ''
followed by an unique identifier (e.g. bert-base-uncased0 and bert-base-uncased1).
* Added an option to pretrain the context encoder, using MLM objective.
* Added beam search.
* New embedding option: XLM-R (XLM trained with Roberta).
* New task: paraphrasing with GPT2. This is not fully integrated with the other tasks yet,
but it will in the future.
* New command "genienlp export" can be used to save a trained model for inference.
Incompatible changes:
* The --save flag is now required when calling train