Add get_sequence_lengths method to HealthDataset and create extract_sequence_lengths script for per-patient sequence length extraction

dataset.py

@@ -88,6 +88,17 @@ class HealthDataset(Dataset):
     def __len__(self) -> int:
         return len(self.patient_ids)
 
+    def get_sequence_lengths(self) -> List[int]:
+        """Return the sequence length for each patient.
+
+        Lengths correspond to what :meth:`__getitem__` returns: the number of
+        patient events plus the inserted DOA event.
+
+        Returns:
+            List[int]: Sequence lengths aligned with dataset indices.
+        """
+        return [len(self.patient_events.get(pid, ())) + 1 for pid in self.patient_ids]
+
     def __getitem__(self, idx):
         if self._cache_event_tensors:
             cached_e = self._cached_event_tensors[idx]