message: [tf.data] Mark DebugString() as const.
  By marking DebugString() as const we can make some error messages more descriptive. Because DatasetIterator marks the return value of the dataset() function const, DebugString() cannot be called.
diff: @@ -81,7 +81,7 @@ class KafkaDatasetOp : public DatasetOpKernel {
  return *shapes;
  }
  - string DebugString() override { return "KafkaDatasetOp::Dataset"; }
  + string DebugString() const override { return "KafkaDatasetOp::Dataset"; }
  protected:
  Status AsGraphDefInternal(DatasetGraphDefBuilder* b,
  