limit num_proc when saving datasets to disk (#2948) [skip ci]

* limit num_proc when saving datasets to disk

* enforce at least 1 in case it rounds down to 0, and sane divisor is at least 8 rows per worker to save

* update fixtures with dataset processes since that should never be NoneType

* improve reusability for tests

tests/test_packed_dataset.py

@@ -99,6 +99,7 @@ class TestPacking(unittest.TestCase):
                         "type": "alpaca",
                     },
                 ],
+                "dataset_processes": 4,
                 "num_epochs": 1,
                 "max_steps": 20,
                 "save_steps": 10,