[DATA] Supports hb.data.Iterator

This patch introduces `hb.data.Iterator`, which optimizes cross-device
transfers of inputs.

Example:

```
iterator = tf.data.make_one_shot_iterator(ds)
  iterator = hb.data.Iterator(iterator)
  features = iterator.get_next()
  hooks.append(hb.data.Iterator.Hook())
```

Author: 2sin18

…ples/ranking/train_criteo_terabyte_v1.py examples/ranking/train_criteo_terabyte.pyexamples/ranking/train_criteo_terabyte_v1.py renamed to examples/ranking/train_criteo_terabyte.py examples/ranking/train_criteo_terabyte_v1.py renamed to examples/ranking/train_criteo_terabyte.py

@@ -194,7 +194,7 @@ def main(args):
       while not sess.should_stop():
         sess.run(train_op)
 
-  hb.saved_model.export(
+  hb.train.export(
     args.savedmodel_dir,
     tf.train.latest_checkpoint(args.output_dir),
     model.predict)

examples/ranking/tutorial.ipynb

@@ -26,6 +26,7 @@
 from hybridbackend.tensorflow.data.dataframe import unbatch_and_to_sparse
 from hybridbackend.tensorflow.data.parquet.dataset import ParquetDataset
 from hybridbackend.tensorflow.data.parquet.dataset import read_parquet
+from hybridbackend.tensorflow.data.prefetch.ops import Iterator
 from hybridbackend.tensorflow.data.rebatch.dataset import RebatchDataset
 from hybridbackend.tensorflow.data.rebatch.dataset import rebatch
 

hybridbackend/tensorflow/data/__init__.py

@@ -13,12 +13,9 @@
 # limitations under the License.
 # =============================================================================
 
-r'''HybridBackend Layers.
+r'''Prefetching related classes and functions.
 '''
 
 from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
-
-from hybridbackend.tensorflow.feature_column.dense_features import \
-  DenseFeatures