Fix dead split parameter in PackedStreamingDataset._load_dataset

Use self.split instead of self.config.dataset_split so the split
argument passed to the constructor is actually forwarded to load_dataset.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

llm_lab/data/dataset.py

@@ -61,7 +61,7 @@ class PackedStreamingDataset(IterableDataset):
         ds = load_dataset(
             self.config.dataset_name,
             name=self.config.dataset_subset,
-            split=self.config.dataset_split,
+            split=self.split,
             streaming=True,         # Key: streaming mode
             trust_remote_code=True,
         )