message: Return early if no whitelist ops found
diff: @@ -1240,6 +1240,11 @@ Status AutoMixedPrecisionImpl::Optimize() {
  AddWhitelistOps(&white_set);
  VLOG(2) << "Finished pass 1";
  + if (white_set.empty()) {
  + LOG(INFO) << "No whitelist ops found, nothing to do";
  + return Status::OK();
  + }
  +
  absl::flat_hash_set<int> black_set;
  VLOG(2) << "Beginning pass 2 to propagate black forwards from blacklist ops "
  "through clear/graylist ops";
  