!1907 fix pynative param bug

Merge pull request !1907 from flywind/fix_pynative_bug

mindspore/nn/cell.py

@@ -202,9 +202,13 @@ class Cell:
             out = self.compile_and_run(*inputs)
             return out
         self.init_parameters_data()
+        orign_grad = []
         if self.requires_grad is True:
             _pynative_exec.set_grad_flag(True)
             _pynative_exec.new_graph(self, *inputs)
+            for cell in self.cells():
+                orign_grad.append(cell.requires_grad)
+                cell.set_grad(True)
         else:
             _pynative_exec.set_grad_flag(False)
         if self.enable_hook:
@@ -215,6 +219,8 @@ class Cell:
             output = output.data
         if self.requires_grad is True:
             _pynative_exec.end_graph(self, output, *inputs)
+            for i, cell in enumerate(self.cells()):
+                cell.set_grad(orign_grad[i])
         self._is_run = True
         return output
 
@@ -744,7 +750,7 @@ class Cell:
         return self
 
     def set_grad(self, mode=True):
-        self.add_flags_recursive(requires_grad=mode)
+        self.requires_grad = mode
         return self
 
     def set_train(self, mode=True):