fix_MultiLangServer_bug and encryption doc

python/examples/encryption/README.md

@@ -12,6 +12,7 @@ sh get_data.sh
 
 ## Encrypt Model
 
+The `paddlepaddle` package is used in this example, you may need to download the corresponding package(`pip install paddlepaddle`).
 ```
 python encrypt.py
 ```

python/examples/encryption/README_CN.md

@@ -11,7 +11,7 @@ sh get_data.sh
 ```
 
 ## 模型加密
-
+本示例中使用了`paddlepaddle`包中的模块，需要进行下载（`pip install paddlepaddle`）。
 ```
 python encrypt.py
 ```

python/paddle_serving_server_gpu/__init__.py

@@ -758,6 +758,9 @@ class MultiLangServer(object):
         self.concurrency_ = concurrency
         self.bserver_.set_max_concurrency(concurrency)
 
+    def set_device(self, device="cpu"):
+        self.device = device
+
     def set_num_threads(self, threads):
         self.worker_num_ = threads
         self.bserver_.set_num_threads(threads)