Faster HF dataset iteration in docs (#1414)

* Faster HF dataset iteration in docs

* Nit

bindings/python/tests/documentation/test_tutorial_train_from_iterators.py

@@ -70,8 +70,10 @@ def test_datasets(self):
 
         # START def_batch_iterator
         def batch_iterator(batch_size=1000):
-            for i in range(0, len(dataset), batch_size):
-                yield dataset[i : i + batch_size]["text"]
+            # Only keep the text column to avoid decoding the rest of the columns unnecessarily
+            tok_dataset = dataset.select_columns("text")
+            for batch in tok_dataset.iter(batch_size):
+                yield batch["text"]
 
         # END def_batch_iterator