[tf.data] Autotune prefetch buffer sizes

In order to make it easier for tf.data users to achieve
high performance with their input pipelines, this change adds
the ability for the prefetch op to automatically tune its buffer
size.

To use the auto-tuning configuration of the `prefetch` transformation,
simply skip passing in a buffer size. Example:

```python
dataset = # ...
dataset = dataset.prefetch()  # Look ma, no buffer value req'd!
```
PiperOrigin-RevId: 190829736