message: [tf.data] Update `DatasetBase::DebugString()` to be const in the docs.
diff: @@ -124,7 +124,7 @@ class MyReaderDatasetOp : public DatasetOpKernel {
  return *shapes;
  }
  - string DebugString() override { return "MyReaderDatasetOp::Dataset"; }
  + string DebugString() const override { return "MyReaderDatasetOp::Dataset"; }
  protected:
  // Optional: Implementation of `GraphDef` serialization for this dataset.
  