boundaries.

*   XLA compilation:

    *   `tf.function(experimental_compile=True)` has become a stable API,
        renamed `tf.function(jit_compile=True)`.
    *   XLA can now compile MirroredStrategy: the step function passed
        to`strategy.run` can now be annoted with `jit_compile=True`.

*   `tf.distribute`:

    *   Rename `experimental_prefetch_to_device` in `tf.distribute.InputOptions`