[generate] do_sample default back to False (#3298)

* change do_samples back

* None better default as boolean

* adapt do_sample to True in test example

* make style

tests/test_modeling_bart.py

@@ -285,7 +285,12 @@ class BartHeadTests(unittest.TestCase):
 
         max_length = 5
         new_input_ids = lm_model.generate(
-            input_ids.clone(), num_return_sequences=1, num_beams=2, no_repeat_ngram_size=3, max_length=max_length
+            input_ids.clone(),
+            do_sample=True,
+            num_return_sequences=1,
+            num_beams=2,
+            no_repeat_ngram_size=3,
+            max_length=max_length,
         )
         self.assertEqual(new_input_ids.shape, (input_ids.shape[0], max_length - 1))
         # TODO(SS): uneven length batches, empty inputs