fix bug of optional_tensor in amp logic (#42561)

Author: zhangbo9674

paddle/fluid/eager/auto_code_generator/final_state_generator/eager_gen.py

@@ -865,7 +865,10 @@ def GenerateForwardDefinition(self, is_inplaced):
                         f"if ({name}.get_ptr() != nullptr) amp_tensors_vector.push_back({{ *({name}.get_ptr()) }});\n"
                     )
                     amp_autocast_optional_list.append(
-                        f"auto NEW_{name} = ({name}.get_ptr() != nullptr) ? paddle::make_optional<const paddle::experimental::Tensor&>(egr::EagerAmpAutoCast(\"{name}\", *({name}.get_ptr()), amp_dst_dtype, op_name)) : {name};\n"
+                        f"auto NEW_{name}_temp_tensor = ({name}.get_ptr() != nullptr) ? egr::EagerAmpAutoCast(\"{name}\", *({name}.get_ptr()), amp_dst_dtype, op_name) : paddle::experimental::Tensor();\n"
+                    )
+                    amp_autocast_optional_list.append(
+                        f"auto NEW_{name} = ({name}.get_ptr() != nullptr) ? paddle::make_optional<const paddle::experimental::Tensor&>(NEW_{name}_temp_tensor) : {name};\n"
                     )
                 else:
                     if is_inplaced and inplace_map and name in inplace_map.keys(