Fix Add None problem

Signed-off-by: zjun <zhangjun0@huawei.com>

mindspore/ccsrc/pipeline/pynative/pynative_execute.cc

@@ -2962,6 +2962,7 @@ void GradExecutor::ClearGrad(const py::object &cell, const py::args &args) {
   if (grad_order_ > 0) {
     --grad_order_;
   }
+  check_graph_cell_id_.clear();
   forward()->node_abs_map().clear();
   ad::CleanRes();
   pipeline::ReclaimOptimizer();

mindspore/ops/composite/multitype_ops/add_impl.py

@@ -227,6 +227,21 @@ def add_rowtensor_tensor(x, y):
     return F.row_tensor_add(x, y)
 
 
+@add.register("None", "None")
+def _none_add_none(x, y):
+    """
+   Adds None and None.
+
+   Args:
+       x (None): x
+       y (None): y
+
+   Returns:
+       None.
+   """
+    return None
+
+
 @_add_backward.register("EnvType", "EnvType")
 def _add_env(x, y):
     """