[tfds] Fix tfds.as_numpy which now returns a reusable iterable.
[tfds] Fix tfds.as_numpy which now returns a reusable iterable.

`next(ds)` -> `next(iter(ds))`

Context: https://github.com/tensorflow/datasets/issues/2270
PiperOrigin-RevId: 325022187

magenta/ddsp,magenta/ddsp