!13565 [Pynative]Fix Pynative AMP Backprop Bugs

From: @chenyijie6
Reviewed-by: @wilfchen,@chujinjin
Signed-off-by: @chujinjin

mindspore/ccsrc/pipeline/pynative/pynative_execute.cc

@@ -897,6 +897,16 @@ py::object ForwardExecutor::DoParamMixPrecisionCast(bool *is_cast, const py::obj
         // Update input for cast struct
         auto cast_struct = cast_struct_pair->second;
         cast_struct->op_inputs[0] = obj;
+        auto grad = this->grad();
+        MS_EXCEPTION_IF_NULL(grad);
+        if (grad->grad_flag()) {
+          // Get forward op index
+          if (!grad->cell_op_info_stack().empty()) {
+            std::string &cell_op_info = grad->cell_op_info_stack().top();
+            cell_op_info += cast_struct->op_index;
+          }
+          grad->op_index_map()[cast_struct->op_name]++;
+        }
         py::object ret = py::none();
         RunOpInner(&ret, cast_struct);
         return ret;