message: [tf.data] In short-circuit `map_and_batch()` perform the batch copies in parallel.
diff: @@ -129,7 +129,10 @@ class MapAndBatchDatasetOp : public UnaryDatasetOpKernel {
  out_tensors->push_back(captured_inputs[indices[i] - num_args]);
  }
  }
  - done(Status::OK());
  + // Run the `done` callback on a threadpool thread, because it will
  + // potentially do a lot of copying work, and we want to run that
  + // concurrently with the next invocation.
  + (*ctx->runner())(std::bind(std::move(done), Status::OK()));
  };
  }
  