fix paddle.var() stop_gradient error (#50063)

python/paddle/tensor/stat.py

@@ -158,6 +158,7 @@ def var(x, axis=None, unbiased=True, keepdim=False, name=None):
     if unbiased:
         one_const = paddle.ones([], x.dtype)
         n = where(n > one_const, n - 1.0, one_const)
+    n.stop_gradient = True
     out /= n
     return out