Add exposure autoencoder pretraining

dataset.py

@@ -42,7 +42,7 @@ def _monthly_exposure_columns() -> list[str]:
 
 
 def _load_readonly_npy(path: Path) -> np.ndarray:
-    arr = np.load(path)
+    arr = np.load(path, mmap_mode="r")
     arr.setflags(write=False)
     return arr