Fix segment Fault from set_value (#31891)

* Avoid raising warning while import paddle

* fix segment fault of set_value

* fix code style

python/paddle/fluid/framework.py

@@ -2006,7 +2006,8 @@ class Variable(object):
                 "paddle.Tensor to a paddle.Tensor, but received {}".format(
                     type(value)))
 
-        self.block.append_op(
+        cur_block = default_main_program().current_block()
+        cur_block.append_op(
             type="set_value", inputs=inputs, outputs={'Out': self}, attrs=attrs)
 
         return self

python/paddle/fluid/tests/unittests/test_set_value_op.py

@@ -106,6 +106,23 @@ class TestSetValueItemSlice4(TestSetValueApi):
         self.data[0:, 1:2, :] = self.value
 
 
+class TestSetValueItemSliceInWhile(TestSetValueApi):
+    def _call_setitem(self, x):
+        def cond(i, x):
+            return i < 1
+
+        def body(i, x):
+            x[i] = self.value
+            i = i + 1
+            return i, x
+
+        i = paddle.zeros(shape=(1, ), dtype='int32')
+        i, x = paddle.fluid.layers.while_loop(cond, body, [i, x])
+
+    def _get_answer(self):
+        self.data[0] = self.value
+
+
 # 1.2.2 step > 1
 class TestSetValueItemSliceStep(TestSetValueApi):
     def set_shape(self):