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_exact_deduplication.py

@@ -210,6 +210,7 @@ class TestDeduplicateRLDataset:
                     ALPACA_MESSAGES_CONFIG_REVISION,
                     ALPACA_MESSAGES_CONFIG_REVISION,
                 ],
+                "dataset_processes": 4,
             }
         )
         yield fixture