message: Replace loop iteration with `chip`
  In unique_op.cc, the ouput tensor was generated
  through loop iteration. It seems that this could be
  improved through Eigen's `chip`.
  The fix addresses this improvement.
diff: @@ -133,11 +133,7 @@ class UniqueOp : public OpKernel {
  auto Tout = output->shaped<T, 3>(new_sizes);
  for (auto it : uniq) {
  - for (int64 i = 0; i < Tin.dimension(0); i++) {
  - for (int64 j = 0; j < Tin.dimension(2); j++) {
  - Tout(i, it.second, j) = Tin(i, it.first, j);
  - }
  - }
  + Tout.chip(it.second, 1) = Tin.chip(it.first, 1);
  }
  if (num_outputs() > 2) {
  