*   Updates to Preprocessing layers API for consistency and clarity:
        *   `StringLookup` and `IntegerLookup` default for `mask_token` changed
            to `None`. This matches the default masking behavior of `Hashing`
            and `Embedding` layers. To keep existing behavior, pass
            `mask_token=""` during layer creation.
        *   Renamed `"binary"` output mode to `"multi_hot"` for