set defaults to cache_dir so old behavior is preserved

src/levanter/data/text.py

@@ -577,6 +577,8 @@ def tagged_eval_sets(
 class LMDatasetConfig(LMDatasetSourceConfig, LMTaskConfig):
     """This class supports loading data both from HF Datasets and from a raw dataset of jsonl urls"""
 
+    cache_dir: Optional[str] = "cache/"
+
     def train_set(
         self, seq_len: int, monitors: Union[bool, List[MetricsMonitor]] = True, *, key: Optional[PRNGKeyArray] = None
     ) -> AsyncDataset[np.ndarray]:
@@ -705,6 +707,8 @@ def _convert_id_to_token(self, index: int) -> str:
 class LMMixtureDatasetConfig(LMTaskConfig):
     """This class represents a mixture of datasets with their associated weights."""
 
+    cache_dir: Optional[str] = "cache/"
+
     # data source configs and weights
     configs: Dict[str, LMDatasetSourceConfig] = field(default_factory=dict)
     """ configuration of each dataset source (urls, hf dataset id, etc.) """