message: [tf.data] Use core::ScopedUnref to avoid resource leakage.
  If for whatever reason iterator_resource->set_iterator did not return Status::OK(), we would leak a reference on the iterator_resource. With this change, we won't leak the resource.
diff: @@ -584,9 +584,9 @@ class MakeIteratorOp : public OpKernel {
  IteratorResource* iterator_resource;
  OP_REQUIRES_OK(
  ctx, LookupResource(ctx, HandleFromInput(ctx, 1), &iterator_resource));
  + core::ScopedUnref unref(iterator_resource);
  OP_REQUIRES_OK(ctx, iterator_resource->set_iterator(
  dataset->MakeIterator("Iterator")));
  - iterator_resource->Unref();
  }
  };
  