[Docs] adding new backends when using MMDeploy as a third package (#482)

* update doc

* refine expression

* cn doc

Author: AllentDan

docs/en/tutorials/how_to_support_new_backends.md

@@ -229,3 +229,20 @@ Although the backend engines are usually implemented in C/C++, it is convenient
     ```
 
 5. Add docstring and unit tests for new code :).
+
+
+### Support new backends using MMDeploy as a third party
+Previous parts show how to add a new backend in MMDeploy, which requires changing its source codes. However, if we treat MMDeploy as a third party, the methods above are no longer efficient. To this end, adding a new backend requires us pre-install another package named `aenum`. We can install it directly through `pip install aenum`.
+
+After installing `aenum` successfully, we can use it to add a new backend through:
+```python
+from mmdeploy.utils.constants import Backend
+from aenum import extend_enum
+
+try:
+    Backend.get('backend_name')
+except Exception:
+    extend_enum(Backend, 'BACKEND', 'backend_name')
+```
+
+We can run the codes above before we use the rewrite logic of MMDeploy.

docs/zh_cn/tutorials/how_to_support_new_backends.md

@@ -229,3 +229,19 @@ MMDeploy 中的后端必须支持 ONNX，因此后端能直接加载“.onnx”
     ```
 
 5. 为新后端引擎代码添加相关注释和单元测试 :).
+
+
+### 将MMDeploy作为第三方库时添加新后端
+前面的部分展示了如何在 MMDeploy 中添加新的后端，这需要更改其源代码。但是，如果我们将 MMDeploy 视为第三方，则上述方法不再有效。为此，添加一个新的后端需要我们预先安装另一个名为 `aenum` 的包。我们可以直接通过`pip install aenum`进行安装。
+
+成功安装 `aenum` 后，我们可以通过以下方式使用它来添加新的后端：
+```python
+from mmdeploy.utils.constants import Backend
+from aenum import extend_enum
+
+try:
+    Backend.get('backend_name')
+except Exception:
+    extend_enum(Backend, 'BACKEND', 'backend_name')
+```
+我们可以在使用 MMDeploy 的重写逻辑之前运行上面的代码，这就完成了新后端的添加。