Always enable local rpc backend (#4505)

* always enable local

* add static_assert

* add log info

Co-authored-by: oneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>

CMakeLists.txt

@@ -66,14 +66,8 @@ if (RPC_BACKEND MATCHES "GRPC")
   message("-- RPC backend enabled: gRPC")
   set(SUPPORTED_RPC_BACKEND_FOUND 1)
 endif()
-if (RPC_BACKEND MATCHES "LOCAL")
-  add_definitions(-DRPC_BACKEND_LOCAL)
-  message("-- RPC backend enabled: local")
-  set(SUPPORTED_RPC_BACKEND_FOUND 1)
-endif()
-if(NOT SUPPORTED_RPC_BACKEND_FOUND)
-  message(FATAL_ERROR "RPC_BACKEND: ${RPC_BACKEND} not supported")
-endif()
+add_definitions(-DRPC_BACKEND_LOCAL)
+message("-- RPC backend enabled: local")
 enable_testing()
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)

oneflow/core/job/env_global_objects_scope.cpp

@@ -97,15 +97,16 @@ Maybe<void> EnvGlobalObjectsScope::Init(const EnvProto& env_proto) {
     auto* local_manager = new LocalRpcManager();
     Global<RpcManager>::SetAllocated(local_manager);
 #else
-    UNIMPLEMENTED();
+    static_assert(false, "requires rpc backend local to run oneflow in single processs");
 #endif  // RPC_BACKEND_LOCAL
   } else /*multi process, multi machine*/ {
 #ifdef RPC_BACKEND_GRPC
     LOG(ERROR) << "using rpc backend: gRPC";
     auto* grpc_manager = new GrpcRpcManager();
     Global<RpcManager>::SetAllocated(grpc_manager);
 #else
-    UNIMPLEMENTED();
+    UNIMPLEMENTED() << "to run distributed oneflow, you must enable at least one multi-node rpc "
+                       "backend by adding cmake argument, for instance: -DRPC_BACKEND=GRPC";
 #endif  // RPC_BACKEND_GRPC
   }
   CHECK_JUST(Global<RpcManager>::Get()->CreateServer());