[tf.data] Fix memory leak when not all elements of a `Dataset.from_generator()` are...

[tf.data] Fix memory leak when not all elements of a `Dataset.from_generator()` are consumed.

This change introduces a new C++ Dataset implementation
(`GeneratorDataset`) that takes three functions:

1. An initialization function that is called before the first use.
2. A "get next" function that is called to produce the elements, until a call
   raises the OutOfRange error.
3. A finalization function that is called before the iterator is destroyed.

Previously, the generator state would only be cleaned up if the caller
consumed *every* element of the generator. In the new version, the
finalization function ensures that the Python-side state of the
generator is released regardless of how the iterator is disposed.

Fixes #16163.

PiperOrigin-RevId: 186360401