adding in missing imports to code in doc (#44203)

#44093 broke one of the data doc tests that was not run on premerge (example here: https://buildkite.com/ray-project/postmerge/builds/3645). This adds missing imports to fix that.

Signed-off-by: Matthew Owen <mowen@anyscale.com>

doc/source/data/transforming-data.rst

@@ -183,7 +183,9 @@ pandas DataFrame or a dictionary with string keys and NumPy ndarrays values. For
 
 .. testcode::
 
-    def fn(batch: pandas.DataFrame) -> pandas.DataFrame:
+    import pandas as pd
+
+    def fn(batch: pd.DataFrame) -> pd.DataFrame:
         # modify batch
         batch = ...
 
@@ -196,6 +198,8 @@ In this case, your function would look like:
 
 .. testcode::
 
+    import numpy as np
+
     def fn(batch: Dict[str, np.ndarray]) -> Iterator[Dict[str, np.ndarray]]:
         # yield the same batch multiple times
         for _ in range(10):