refine add_n_op's error message (#10411)

Before this commit, error messge is: **inconsistent tensor size,
expected all tensor to have the same number of elements, but got 640000
and 742400**
After this commit, error message is **inconsistent tensor size, expected
all tensor to have the same number of elements, but got (1,256,50,50)
and (1,256,50,58)**

oneflow/user/ops/add_n_op.cpp

@@ -26,8 +26,8 @@ namespace oneflow {
     if (in_0.shape().NumAxes() > 0 && cur_in.shape().NumAxes() > 0) {
       CHECK_EQ_OR_RETURN(in_0.shape(), cur_in.shape())
           << Error::RuntimeError()
-          << "inconsistent tensor size, expected all tensor to have the same number of elements, "
-          << "but got " << in_0.shape().elem_cnt() << " and " << cur_in.shape().elem_cnt();
+          << "inconsistent tensor size, expected all tensor to have the same shapes, "
+          << "but got " << in_0.shape().ToString() << " and " << cur_in.shape().ToString();
     }
   }
   out->set_shape(in_0.shape());