message: Mark tf.write_file as stateful. Fixes
diff: @@ -474,6 +474,7 @@ REGISTER_OP("ReadFile")
  REGISTER_OP("WriteFile")
  .Input("filename: string")
  .Input("contents: string")
  + .SetIsStateful()
  .SetShapeFn([](InferenceContext* c) {
  ShapeHandle unused;
  TF_RETURN_IF_ERROR(c->WithRank(c->input(0), 0, &unused));
  