Fix: Address review comments on PandasBatches and docstring

- Reinstated 'Iterator[pd.DataFrame]' inheritance for 'PandasBatches' in 'bigframes/core/blocks.py'.
- Removed internal type hint 'bigframes.core.blocks.PandasBatches:' from 'to_pandas_batches' docstring in 'bigframes/dataframe.py' to avoid exposing internal types in public documentation.

Author: shuoweil

bigframes/core/blocks.py

@@ -99,7 +99,7 @@
 
 
 @dataclasses.dataclass
-class PandasBatches:
+class PandasBatches(Iterator[pd.DataFrame]):
     """Interface for mutable objects with state represented by a block value object."""
 
     def __init__(

bigframes/dataframe.py

@@ -1925,10 +1925,9 @@ def to_pandas_batches(
                 over the default size limit of 10 GB.
 
         Returns:
-            bigframes.core.blocks.PandasBatches:
-                An iterable of smaller dataframes which combine to
-                form the original dataframe. Results stream from bigquery,
-                see https://cloud.google.com/python/docs/reference/bigquery/latest/google.cloud.bigquery.table.RowIterator#google_cloud_bigquery_table_RowIterator_to_arrow_iterable
+            An iterable of smaller dataframes which combine to
+            form the original dataframe. Results stream from bigquery,
+            see https://cloud.google.com/python/docs/reference/bigquery/latest/google.cloud.bigquery.table.RowIterator#google_cloud_bigquery_table_RowIterator_to_arrow_iterable
         """
         return self._to_pandas_batches(
             page_size=page_size,