message: Fix meaningless "const" on return type.
  Because this file is included in lots of places, this generates a large number of warnings during compilation.
diff: @@ -184,9 +184,7 @@ class DeviceBase {
  virtual ScopedAllocatorMgr* GetScopedAllocatorMgr() const { return nullptr; }
  - const bool has_eigen_cpu_device() const {
  - return !eigen_cpu_devices_.empty();
  - }
  + bool has_eigen_cpu_device() const { return !eigen_cpu_devices_.empty(); }
  virtual const Eigen::ThreadPoolDevice* eigen_cpu_device();
  