【Eager】Fix error raise (#51963)

* allow return none when stop_gradient=True

* remove useless code

* refine code

* refine code

* fix test cast

* change more test

* add more tests

* fix error msg in pylayer

paddle/fluid/eager/pylayer/py_layer_node.cc

@@ -166,8 +166,8 @@ GradNodePyLayer::operator()(
             grad_out.push_back({});
           } else {
             PADDLE_THROW(phi::errors::InvalidArgument(
-                "argument must be "
-                "Tensor or None, but got %s",
+                "We can only support Tensor or None for backward output, "
+                ", but got %s, please check your PyLayer code and make it fits",
                 reinterpret_cast<PyTypeObject*>(obj->ob_type)->tp_name));
           }
         }