Clean up naming and usage of internal functions and classes in dataset op files.
Clean up naming and usage of internal functions and classes in dataset op files.

- Added leading underscores to the names of dataset op functions and classes to clarify that they're private and not meant to be depended on externally.
- Replaced unnecessary internal uses of dataset op functions and classes with calls to the public API.
- Cleaned up unnecessary Python 2-style calls to `super` in dataset op class constructors.

PiperOrigin-RevId: 489573048

keras-team/keras,keras-team/keras