[Doc]: Fix CPU install docs: force torch-backend=cpu to avoid GPU torchvision errors (#24033)

Signed-off-by: Kay Yan <kay.yan@daocloud.io>

docs/getting_started/installation/cpu/build.inc.md

@@ -16,8 +16,8 @@ cd vllm_source
 Third, install required dependencies:
 
 ```bash
-uv pip install -r requirements/cpu-build.txt --torch-backend auto
-uv pip install -r requirements/cpu.txt --torch-backend auto
+uv pip install -r requirements/cpu-build.txt --torch-backend cpu
+uv pip install -r requirements/cpu.txt --torch-backend cpu
 ```
 
 ??? console "pip"