Fix gradients (#20857)

* fix_gradients

* fix_gradients, test=develop

python/paddle/fluid/backward.py

@@ -1247,9 +1247,13 @@ def calc_gradient(targets, inputs, target_gradients=None, no_grad_set=None):
         target = targets[i]
         if grad is None:
             grad_name = _append_grad_suffix_(target.name)
-            target_shape = paddle.fluid.layers.shape(target)
+            target_shape = target.name + '_shape'
+            block.desc.append_op().copy_from(
+                _create_op_desc_("shape", {'Input': [target.name]},
+                                 {"Out": [target_shape]}, {}))
+            input_grad_names_set.add(target_shape)
             op_desc = _create_op_desc_("fill_constant",
-                                       {"ShapeTensor": [target_shape.name]},
+                                       {"ShapeTensor": [target_shape]},
                                        {"Out": [grad_name]}, {
                                            "shape": target.shape,
                                            "value": 1.0,