We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
[PS-1]Add new RPC services definition in elasticdl.proto according to PS design
[PS design](https://github.com/sql-machine-learning/elasticdl/blob/develop/docs/designs/ps_design.md#rpc-definition) adds some new RPC services.

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `elasticdl/python/ps/servicer.py`
Content:
```
1 from google.protobuf import empty_pb2
2 
3 from elasticdl.proto import elasticdl_pb2_grpc
4 
5 
6 class PserverServicer(elasticdl_pb2_grpc.PserverServicer):
7     """PS service implementation"""
8 
9     def __init__(
10         self,
11         parameters,
12         grads_to_wait,
13         optimizer,
14         lr_staleness_modulation=False,
15         use_async=False,
16     ):
17         self._parameters = parameters
18         self._grads_to_wait = grads_to_wait
19         self._optimizer = optimizer
20         self._lr_staleness_modulation = lr_staleness_modulation
21         self._use_async = use_async
22         self._version = 0
23 
24     def pull_variable(self, request, _):
25         # TODO: implement this RPC service
26         return empty_pb2.Empty()
27 
28     def pull_embedding_vector(self, request, _):
29         # TODO: implement this RPC service
30         return empty_pb2.Empty()
31 
32     def push_model(self, request, _):
33         # TODO: implement this RPC service
34         return empty_pb2.Empty()
35 
36     def push_gradient(self, request, _):
37         # TODO: implement this RPC service
38         return empty_pb2.Empty()
39
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/elasticdl/python/ps/servicer.py b/elasticdl/python/ps/servicer.py
--- a/elasticdl/python/ps/servicer.py
+++ b/elasticdl/python/ps/servicer.py
@@ -1,6 +1,6 @@
 from google.protobuf import empty_pb2
 
-from elasticdl.proto import elasticdl_pb2_grpc
+from elasticdl.proto import elasticdl_pb2, elasticdl_pb2_grpc
 
 
 class PserverServicer(elasticdl_pb2_grpc.PserverServicer):
@@ -23,11 +23,11 @@
 
     def pull_variable(self, request, _):
         # TODO: implement this RPC service
-        return empty_pb2.Empty()
+        return elasticdl_pb2.PullVariableResponse()
 
     def pull_embedding_vector(self, request, _):
         # TODO: implement this RPC service
-        return empty_pb2.Empty()
+        return elasticdl_pb2.Tensor()
 
     def push_model(self, request, _):
         # TODO: implement this RPC service
@@ -35,4 +35,4 @@
 
     def push_gradient(self, request, _):
         # TODO: implement this RPC service
-        return empty_pb2.Empty()
+        return elasticdl_pb2.PushGradientResponse()

{"golden_diff": "diff --git a/elasticdl/python/ps/servicer.py b/elasticdl/python/ps/servicer.py\n--- a/elasticdl/python/ps/servicer.py\n+++ b/elasticdl/python/ps/servicer.py\n@@ -1,6 +1,6 @@\n from google.protobuf import empty_pb2\n \n-from elasticdl.proto import elasticdl_pb2_grpc\n+from elasticdl.proto import elasticdl_pb2, elasticdl_pb2_grpc\n \n \n class PserverServicer(elasticdl_pb2_grpc.PserverServicer):\n@@ -23,11 +23,11 @@\n \n     def pull_variable(self, request, _):\n         # TODO: implement this RPC service\n-        return empty_pb2.Empty()\n+        return elasticdl_pb2.PullVariableResponse()\n \n     def pull_embedding_vector(self, request, _):\n         # TODO: implement this RPC service\n-        return empty_pb2.Empty()\n+        return elasticdl_pb2.Tensor()\n \n     def push_model(self, request, _):\n         # TODO: implement this RPC service\n@@ -35,4 +35,4 @@\n \n     def push_gradient(self, request, _):\n         # TODO: implement this RPC service\n-        return empty_pb2.Empty()\n+        return elasticdl_pb2.PushGradientResponse()\n", "issue": "[PS-1]Add new RPC services definition in elasticdl.proto according to PS design\n[PS design](https://github.com/sql-machine-learning/elasticdl/blob/develop/docs/designs/ps_design.md#rpc-definition) adds some new RPC services.\n", "before_files": [{"content": "from google.protobuf import empty_pb2\n\nfrom elasticdl.proto import elasticdl_pb2_grpc\n\n\nclass PserverServicer(elasticdl_pb2_grpc.PserverServicer):\n    \"\"\"PS service implementation\"\"\"\n\n    def __init__(\n        self,\n        parameters,\n        grads_to_wait,\n        optimizer,\n        lr_staleness_modulation=False,\n        use_async=False,\n    ):\n        self._parameters = parameters\n        self._grads_to_wait = grads_to_wait\n        self._optimizer = optimizer\n        self._lr_staleness_modulation = lr_staleness_modulation\n        self._use_async = use_async\n        self._version = 0\n\n    def pull_variable(self, request, _):\n        # TODO: implement this RPC service\n        return empty_pb2.Empty()\n\n    def pull_embedding_vector(self, request, _):\n        # TODO: implement this RPC service\n        return empty_pb2.Empty()\n\n    def push_model(self, request, _):\n        # TODO: implement this RPC service\n        return empty_pb2.Empty()\n\n    def push_gradient(self, request, _):\n        # TODO: implement this RPC service\n        return empty_pb2.Empty()\n", "path": "elasticdl/python/ps/servicer.py"}], "after_files": [{"content": "from google.protobuf import empty_pb2\n\nfrom elasticdl.proto import elasticdl_pb2, elasticdl_pb2_grpc\n\n\nclass PserverServicer(elasticdl_pb2_grpc.PserverServicer):\n    \"\"\"PS service implementation\"\"\"\n\n    def __init__(\n        self,\n        parameters,\n        grads_to_wait,\n        optimizer,\n        lr_staleness_modulation=False,\n        use_async=False,\n    ):\n        self._parameters = parameters\n        self._grads_to_wait = grads_to_wait\n        self._optimizer = optimizer\n        self._lr_staleness_modulation = lr_staleness_modulation\n        self._use_async = use_async\n        self._version = 0\n\n    def pull_variable(self, request, _):\n        # TODO: implement this RPC service\n        return elasticdl_pb2.PullVariableResponse()\n\n    def pull_embedding_vector(self, request, _):\n        # TODO: implement this RPC service\n        return elasticdl_pb2.Tensor()\n\n    def push_model(self, request, _):\n        # TODO: implement this RPC service\n        return empty_pb2.Empty()\n\n    def push_gradient(self, request, _):\n        # TODO: implement this RPC service\n        return elasticdl_pb2.PushGradientResponse()\n", "path": "elasticdl/python/ps/servicer.py"}]}