fix sft bug caused by exemplar

tldr exemplar is a schema for data. I was storing the sequences and lengths with np arrays as schema objects and one of them had dim 0 which is a scalar and invalid for arrays

src/levanter/data/text.py

@@ -725,7 +725,8 @@ def mk_supervised_dataset(config: LMSupervisedDatasetConfig, tokenizer: PreTrain
     input_field = config.input_field
     output_field = config.output_field
 
-    output_exemplar = {"input_ids": np.zeros((0,), dtype=np.int32), "sources_len": np.zeros((), dtype=np.int32)}
+
+    output_exemplar = {"input_ids": np.zeros((0,), dtype=np.int32), "sources_len": np.zeros((0,), dtype=np.int32)}
 
     # Use the same preprocessing as before
     dataset = dataset.map_batches(