Get the argmax of the class predictions.
The predicted classes can be an arbitrary set of non-negative integer classes,
but in our current sentiment models, the values used are on a 5-point
scale of 0 = very negative, 1 = negative, 2 = neutral, 3 = positive,
and 4 = very positive.
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.