[Distributed] fix release grad on moe model (#74972)

Author: SylarTiaNII

python/paddle/distributed/fleet/meta_optimizers/dygraph_optimizer/dygraph_sharding_optimizer.py

@@ -348,6 +348,13 @@ def reduce_gradients(self, parameter_list, hcg):
         with framework.no_grad():
             for param in parameter_list:
                 g_var = self._get_param_grad(param)
+                if g_var is None:
+                    if hasattr(param, "main_grad"):
+                        g_var = paddle.zeros_like(param, dtype=paddle.float32)
+                        param.main_grad = g_var
+                    else:
+                        g_var = paddle.zeros_like(param, dtype=param.dtype)
+                        param.grad = g_var
                 if g_var is not None:
                     reduce_op = ReduceOp.AVG
                     if not self.use_reduce_avg:

python/paddle/distributed/fleet/utils/tensor_fusion_helper.py

@@ -619,22 +619,19 @@ def _copy_grad_to_buffer(self, param):
             )
 
         grad_var = param.main_grad if self.use_main_grad else param.grad
-        assert grad_var is not None, (
-            f"The current parameter[{param.name}] has no gradient, its stop_grdient is {param.stop_gradient}"
-        )
-        grad_var.stop_gradient = True
-        grad_var.flatten_()
 
-        tmp_var.add_(grad_var)
-        tmp_var.get_tensor()._set_dims(param.shape)
+        if grad_var is not None:
+            grad_var.stop_gradient = True
+            grad_var.flatten_()
+            tmp_var.add_(grad_var)
+            grad_var._clear()
 
+        tmp_var.get_tensor()._set_dims(param.shape)
         if self.use_main_grad:
-            param.main_grad._clear()
             if not self._free_grads_in_comm:
                 param.main_grad = tmp_var
                 param.main_grad.name = "main_grad@" + param.name
         else:
-            param.grad._clear()
             if not self._free_grads_in_comm:
                 param._copy_gradient_from(tmp_var)