    def apply_token_indexers(self, instance: Instance) -> None:
        """
        If `Instance`s created by this reader contain `TextField`s without `token_indexers`,
        this method can be overriden to set the `token_indexers` of those fields.

        E.g. if you have you have `"source"` `TextField`, you could implement this method like this:

        ```python
        def apply_token_indexers(self, instance: Instance) -> None:
            instance["source"].token_indexers = self._token_indexers
        ```

        If your `TextField`s are wrapped in a `ListField`, you can access them via `field_list`.
        E.g. if you had a `"source"` field of `ListField[TextField]` objects, you could:

        ```python
        for text_field in instance["source"].field_list:
            text_field.token_indexers = self._token_indexers
        ```
        """
        pass
If `Instance`s created by this reader contain `TextField`s without `token_indexers`,
this method can be overriden to set the `token_indexers` of those fields.

E.g. if you have you have `"source"` `TextField`, you could implement this method like this:

```python
def apply_token_indexers(self, instance: Instance) -> None:
instance["source"].token_indexers = self._token_indexers
```

If your `TextField`s are wrapped in a `ListField`, you can access them via `field_list`.
E.g. if you had a `"source"` field of `ListField[TextField]` objects, you could:

```python
for text_field in instance["source"].field_list:
text_field.token_indexers = self._token_indexers
```
allennlp/data/dataset_readers/dataset_reader.py