message: Silence compilation warning in allocator.h
  Some builds report the following warning (even though LOG(FATAL) is terminal):
  ```
  ./tensorflow/core/framework/device_base.h(188): warning: missing return statement at end of non-void function "tensorflow::DeviceBase::GetScopedAllocator"
  ```
diff: @@ -185,6 +185,7 @@ class DeviceBase {
  virtual Allocator* GetScopedAllocator(AllocatorAttributes attr,
  int64 step_id) {
  LOG(FATAL) << "Device does not implement GetScopedAllocator()";
  + return nullptr;
  }
  virtual ScopedAllocatorMgr* GetScopedAllocatorMgr() const { return nullptr; }
  