Fix bug for pipeline & add docs

doc/INFERENCE_TO_SERVING.md

-# How to Convert Paddle Inference Model To Paddle Serving Format
-
-([简体中文](./INFERENCE_TO_SERVING_CN.md)|English)
-
-you can use a build-in python module called `paddle_serving_client.convert` to convert it.
-```python
-python -m paddle_serving_client.convert --dirname ./your_inference_model_dir
-```
-Arguments are the same as `inference_model_to_serving` API.
-| Argument | Type | Default | Description |
-|--------------|------|-----------|--------------------------------|
-| `dirname` | str | - | Path of saved model files. Program file and parameter files are saved in this directory. |
-| `serving_server` | str | `"serving_server"` | The path of model files and configuration files for server. |
-| `serving_client` | str | `"serving_client"` | The path of configuration files for client. |
-| `model_filename` | str | None | The name of file to load the inference program. If it is None, the default filename `__model__` will be used. |
-| `params_filename` | str | None | The name of file to load all parameters. It is only used for the case that all parameters were saved in a single binary file. If parameters were saved in separate files, set it as None. |

doc/INFERENCE_TO_SERVING_CN.md

-# 如何从Paddle保存的预测模型转为Paddle Serving格式可部署的模型
-
-([English](./INFERENCE_TO_SERVING.md)|简体中文)
-
-你可以使用Paddle Serving提供的名为`paddle_serving_client.convert`的内置模块进行转换。
-```python
-python -m paddle_serving_client.convert --dirname ./your_inference_model_dir
-```
-模块参数与`inference_model_to_serving`接口参数相同。
-| 参数 | 类型 | 默认值 | 描述 |
-|--------------|------|-----------|--------------------------------|
-| `dirname` | str | - | 需要转换的模型文件存储路径，Program结构文件和参数文件均保存在此目录。|
-| `serving_server` | str | `"serving_server"` | 转换后的模型文件和配置文件的存储路径。默认值为serving_server |
-| `serving_client` | str | `"serving_client"` | 转换后的客户端配置文件存储路径。默认值为serving_client |
-| `model_filename` | str | None | 存储需要转换的模型Inference Program结构的文件名称。如果设置为None，则使用 `__model__` 作为默认的文件名 |
-| `params_filename` | str | None | 存储需要转换的模型所有参数的文件名称。当且仅当所有模型参数被保存在一个单独的>二进制文件中，它才需要被指定。如果模型参数是存储在各自分离的文件中，设置它的值为None |

python/pipeline/channel.py

@@ -767,7 +767,7 @@ class ThreadChannel(Queue.PriorityQueue):
             while self._stop is False and self._consumer_cursors[
                     op_name] - self._base_cursor >= len(self._output_buf):
                 try:
-                    channeldata = self.get(timeout=0)
+                    channeldata = self.get(timeout=0)[1]
                     self._output_buf.append(channeldata)
                     list_values = list(channeldata.values())
                     _LOGGER.debug(