The CoreMap key for getting the token-level named entity tag (e.g., DATE,
PERSON, etc.) from a previous NER tagger. NERFeatureFactory is sensitive to
this tag and will turn the annotations from the previous NER tagger into
new features. This is currently used to implement one level of stacking --
we may later change it to take a list as needed.
This key is typically set on token annotations.
Returns the type associated with this annotation. This method must
return the same class type as its value type parameter. It feels like
one should be able to get away without this method, but because Java
erases the generic type signature, that info disappears at runtime.