Add _mp_fn to run_mae.py for XLA testing (#21551)

Update run_mae.py

examples/pytorch/image-pretraining/run_mae.py

@@ -384,5 +384,10 @@ def main():
         trainer.create_model_card(**kwargs)
 
 
+def _mp_fn(index):
+    # For xla_spawn (TPUs)
+    main()
+
+
 if __name__ == "__main__":
     main()