    def empty_field(self) -> "Field":
        """
        So that `ListField` can pad the number of fields in a list (e.g., the number of answer
        option `TextFields`), we need a representation of an empty field of each type.  This
        returns that.  This will only ever be called when we're to the point of calling
        :func:`as_tensor`, so you don't need to worry about `get_padding_lengths`,
        `count_vocab_items`, etc., being called on this empty field.

        We make this an instance method instead of a static method so that if there is any state
        in the Field, we can copy it over (e.g., the token indexers in `TextField`).
        """
        raise NotImplementedError
So that `ListField` can pad the number of fields in a list (e.g., the number of answer
option `TextFields`), we need a representation of an empty field of each type.  This
returns that.  This will only ever be called when we're to the point of calling
:func:`as_tensor`, so you don't need to worry about `get_padding_lengths`,
`count_vocab_items`, etc., being called on this empty field.

We make this an instance method instead of a static method so that if there is any state
in the Field, we can copy it over (e.g., the token indexers in `TextField`).
allennlp/data/fields/field.py