Move of Dataset API to tf.data.

This also modifies semantics of `dataset.enumerate()`, `dataset.dense_to_sparse_batch()`, `dataset.ignore_errors()` and
`dataset.unbatch()`; which now return a transformation function from `Dataset` to `Dataset`.

Further API `tf.contrib.data.batch_and_drop_remainder()`, `tf.contrib.data.dense_to_sparse_batch()`, `tf.contrib.data.enumerate_dataset()`, `tf.contrib.data.group_by_window()`,  `tf.contrib.data.ignore_errors()`, `tf.contrib.data.read_batch_features()`, `tf.contrib.data.sloppy_interleave()` and `tf.contrib.data.unbatch()` did not move to tf.data.

PiperOrigin-RevId: 169746213