docs: add CPU / GPU docs

Author: SWHL

docs/install_usage/rapidocr/usage.md

@@ -645,27 +645,41 @@ RapidOCR输出包括4种类型：`Union[TextDetOutput, TextClsOutput, TextRecOut
 
     2. 指定PaddlePaddle作为推理引擎
 
-         ```python linenums="1" hl_lines="4"
-         from rapidocr import RapidOCR
+        === CPU版
 
-         # CPU版直接使用
-         engine = RapidOCR(params={"Global.with_paddle": True})
+            ```python linenums="1" hl_lines="4"
+            from rapidocr import RapidOCR
 
-         # GPU版，指定GPU id
-         engine = RapidOCR(
-            params={
-               "Global.with_paddle": True,
-               "EngineConfig.paddlepaddle.use_cuda": True,  # 使用torch GPU版推理
-               "EngineConfig.paddlepaddle.gpu_id": 0,  # 指定GPU id
-            }
-         )
+            # CPU版直接使用
+            engine = RapidOCR(params={"Global.with_paddle": True})
 
-         img_url = "https://github.com/RapidAI/RapidOCR/blob/main/python/tests/test_files/ch_en_num.jpg?raw=true"
-         result = engine(img_url)
-         print(result)
+            img_url = "https://github.com/RapidAI/RapidOCR/blob/main/python/tests/test_files/ch_en_num.jpg?raw=true"
+            result = engine(img_url)
+            print(result)
 
-         result.vis('vis_result.jpg')
-         ```
+            result.vis('vis_result.jpg')
+            ```
+
+        === GPU版
+
+            ```python linenums="1" hl_lines="4"
+            from rapidocr import RapidOCR
+
+            # GPU版，指定GPU id
+            engine = RapidOCR(
+                params={
+                "Global.with_paddle": True,
+                "EngineConfig.paddlepaddle.use_cuda": True,  # 使用torch GPU版推理
+                "EngineConfig.paddlepaddle.gpu_id": 0,  # 指定GPU id
+                }
+            )
+
+            img_url = "https://github.com/RapidAI/RapidOCR/blob/main/python/tests/test_files/ch_en_num.jpg?raw=true"
+            result = engine(img_url)
+            print(result)
+
+            result.vis('vis_result.jpg')
+            ```
 
     3. 查看输出日志。下面日志中打印出了**Using engine_name: paddle**，则证明使用的推理引擎是PaddlePaddle。