Move the redundant numpy() (#40931)

python/paddle/fluid/layers/control_flow.py

@@ -2432,7 +2432,7 @@ def cond(pred, true_fn=None, false_fn=None, name=None):
     """
     if _non_static_mode():
         assert isinstance(pred, Variable), "The pred in cond must be Variable"
-        assert pred.numpy().size == 1, "condition input's numel should be 1"
+        assert pred.size == 1, "condition input's numel should be 1"
         pred = pred.numpy()[0]
         if pred:
             if true_fn is not None: