[tf.data] Fix a memory leak when an iterator is reinitialized many times in a session.

[tf.data] Fix a memory leak when an iterator is reinitialized many times in a session.

Previously, we would instantiate a new function handle for each
function in a dataset each time an iterator on that dataset was
initialized. These would only be deleted at session closure, which
could lead to an apparent leak of memory over the lifetime of session.

PiperOrigin-RevId: 184768730