The CoreMap key for getting relation mentions corresponding to a sentence. Whereas
RelationMentionsAnnotation gives only relations pertaining to a test entity,
AllRelationMentionsAnnotation gives all pairwise relations.
This key is typically set on sentence 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.