I cannot find any sources about the architectures of machine learning models to solve for NER problems. I vaguely knows it is a multiclass classification problem, but how can we format our input to feed into such multiclass classifier? I know the inputs must be annotated corpus, but how can we feed that chunk of pairs of (word, entity label) into the classifier? Or, how do you feature-engineer such corpus to feed into ML models? Or, in general, how can you train a custom NER from scratch with machine learning?