Fix multiple definitions

This commit fixes multiple definitions issues in the caffe2 test harness.
This was caught internally when building on FB Infra.

Author: Nicolas Vasilache

test/caffe2/test_harness-inl.h

@@ -19,10 +19,7 @@ namespace caffe2 {
 
 namespace detail {
 
-std::mutex& RNGMutex() {
-  static std::mutex rng_mutex;
-  return rng_mutex;
-}
+std::mutex& RNGMutex();
 
 template <typename T>
 T* NewTensor(

test/caffe2/test_harness.cc

@@ -16,6 +16,20 @@
 #include "test_harness.h"
 
 namespace caffe2 {
+namespace detail {
+
+std::mutex& RNGMutex() {
+  static std::mutex rng_mutex;
+  return rng_mutex;
+}
+
+} // namespace detail
+
+ReferenceImplementationBuilder MakeDefaultReferenceImplementationBuilder() {
+  return [](const OperatorDef& op_def, NetDef* net_def) {
+    caffe2::ReferenceImplementationRegistry::Append(net_def, op_def);
+  };
+}
 
 void CheckEqual(
     const caffe2::Tensor<caffe2::CPUContext>& Texpected,

test/caffe2/test_harness.h

@@ -87,11 +87,7 @@ at::Tensor MakeAtenTensor(
 using ReferenceImplementationBuilder =
     std::function<void(const OperatorDef& op_def, NetDef* net_def)>;
 
-ReferenceImplementationBuilder MakeDefaultReferenceImplementationBuilder() {
-  return [](const OperatorDef& op_def, NetDef* net_def) {
-    caffe2::ReferenceImplementationRegistry::Append(net_def, op_def);
-  };
-}
+ReferenceImplementationBuilder MakeDefaultReferenceImplementationBuilder();
 
 /// Creates an OperatorDef for a particular Backend
 /// op_name is the name of the operator (e.g. TcOp)