12. A system comprising: one or more computers and one or more storage devices storing instructions which are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: obtaining a training data set comprising at least one training example, the at least one training example being annotated with at least one training example natural language processing tag; adding a training example information retrieval model annotation to the at least one training example in the training data set to obtain an annotated training data set; training a natural language processing model on the annotated training data set to obtain a trained natural language processing model, wherein the operation of training comprises training the natural language processing model based on both the training example natural language processing tag and the training example information retrieval model annotation of the at least one training example; wherein training the natural language processing model on the annotated training data set comprises: extracting information retrieval features from the at least one training example in the annotated training data set based on the information retrieval model annotation, predicting part-of-speech tags for at least one word in the at least one training example, generating a confidence score for the predicted part-of-speech tags, and filtering the predicted part-of-speech tags if the confidence score is below a threshold; receiving a target document comprising text and at least one information retrieval model annotation; and generating, for at least one word in the text, a prediction and an additional confidence score for the prediction with the natural language processing model, wherein the operation of generating the prediction and the additional confidence score comprises using the information retrieval model annotation with the natural language processing model to generate the prediction.
12. A system comprising: one or more computers and one or more storage devices storing instructions which are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: obtaining a training data set comprising at least one training example, the at least one training example being annotated with at least one training example natural language processing tag; adding a training example information retrieval model annotation to the at least one training example in the training data set to obtain an annotated training data set; training a natural language processing model on the annotated training data set to obtain a trained natural language processing model, wherein the operation of training comprises training the natural language processing model based on both the training example natural language processing tag and the training example information retrieval model annotation of the at least one training example; wherein training the natural language processing model on the annotated training data set comprises: extracting information retrieval features from the at least one training example in the annotated training data set based on the information retrieval model annotation, predicting part-of-speech tags for at least one word in the at least one training example, generating a confidence score for the predicted part-of-speech tags, and filtering the predicted part-of-speech tags if the confidence score is below a threshold; receiving a target document comprising text and at least one information retrieval model annotation; and generating, for at least one word in the text, a prediction and an additional confidence score for the prediction with the natural language processing model, wherein the operation of generating the prediction and the additional confidence score comprises using the information retrieval model annotation with the natural language processing model to generate the prediction. 14. The system of claim 12 , wherein the at least one training example comprises text and wherein the at least one training example natural language processing tag comprises a part-of-speech tag.