chore: add an unified server arg for multimodal inputs preprocess config(#12149)

Co-authored-by: bianfeng <bianfeng@pinduoduo.com>
Co-authored-by: Xinyuan Tong <115166877+JustinTong0323@users.noreply.github.com>

docs/advanced_features/server_arguments.md

@@ -156,6 +156,7 @@ Please consult the documentation below and [server_args.py](https://github.com/s
 | `--base-gpu-id` | The base GPU ID to start allocating GPUs from. Useful when running multiple instances on the same machine. | `0` | Type: int |
 | `--gpu-id-step` | The delta between consecutive GPU IDs that are used. For example, setting it to 2 will use GPU 0,2,4,... | `1` | Type: int |
 | `--sleep-on-idle` | Reduce CPU usage when sglang is idle. | `False` | bool flag (set to enable) |
+| `--mm-process-config` | A JSON string for multimodal preprocessing configuration. It can contain keys: `image`, `video`, `audio`. | `{}` |
 
 ## Logging
 | Argument | Description | Defaults | Options |