message: Fix compiler warnings on overridden methods in SessionRef.
diff: @@ -63,14 +63,14 @@ class SessionRef : public Session {
  std::vector<Tensor>* outputs) override;
  Status MakeCallable(const CallableOptions& callable_options,
  - CallableHandle* out_handle);
  + CallableHandle* out_handle) override;
  Status RunCallable(CallableHandle handle,
  const std::vector<Tensor>& feed_tensors,
  std::vector<Tensor>* fetch_tensors,
  - RunMetadata* run_metadata);
  + RunMetadata* run_metadata) override;
  - Status ReleaseCallable(CallableHandle handle);
  + Status ReleaseCallable(CallableHandle handle) override;
  private:
  mutex run_lock_;
  