Remove the requirement that p.file_datasource needs to be entirely constructed before the call to super().__init__.
Remove the requirement that p.file_datasource needs to be entirely constructed
before the call to super().__init__.

By delegating self.CreateChild('datasource') to a helper function, this allows
subclasses to override the function to modify p.file_datasource before the child
is created.

PiperOrigin-RevId: 360550993

tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo