Add the field to the existing fields mapping.
        If we have already indexed the Instance, then we also index `field`, so
        it is necessary to supply the vocab.

def add_field(self, field_name: str, field: Field, vocab: Vocabulary = None) -> None:
        """
        Add the field to the existing fields mapping.
        If we have already indexed the Instance, then we also index `field`, so
        it is necessary to supply the vocab.
        """
        self.fields[field_name] = field
        if self.indexed:
            field.index(vocab)