[IO] Add hb.data.rebatch for resizing batches from tabular data.

This patch introduces `hb.data.rebatch` for resizing batches from tabular data.

**Example**:
```python
import tensorflow as tf
import hybridbackend.tensorflow as hb

ds = hb.data.ParquetDataset(filename, micro_batch_size)
ds = ds.apply(hb.data.rebatch(batch_size))
```

Author: 2sin18

docs/tutorials/data_loading.md

@@ -36,6 +36,7 @@ Text     | `string`
     :special-members: __init__
 .. autofunction:: hybridbackend.tensorflow.data.read_parquet
 .. autofunction:: hybridbackend.tensorflow.data.to_sparse
+.. autofunction:: hybridbackend.tensorflow.data.rebatch
 ```
 
 ## Data Loading Use Cases
@@ -89,6 +90,26 @@ batch = it.get_next()
 ...
 ```
 
+### Read and shuffle samples
+
+```python
+import tensorflow as tf
+import hybridbackend.tensorflow as hb
+filenames = tf.data.Dataset.from_generator(func, tf.string, tf.TensorShape([]))
+fields = [
+    hb.data.DataFrame.Field('A', tf.int64),
+    hb.data.DataFrame.Field('C', tf.int64, ragged_rank=1)]
+ds = filenames.apply(hb.data.read_parquet(1024, fields=fields))
+ds = ds.shuffle(2048 // 256)
+ds = ds.apply(hb.data.rebatch(1024, fields=fields))
+ds = ds.apply(hb.data.to_sparse())
+ds = ds.prefetch(4)
+it = tf.data.make_one_shot_iterator(ds)
+batch = it.get_next()
+# {'a': tensora, 'c': tensorc}
+...
+```
+
 ## Performance
 
 In benchmark for reading 20k samples from 200 columns of a Parquet file,