message: Improve shape function for CudnnRNNParamsSize
  In cudnn_rnn_ops.cc, the CudnnRNNParamsSize does not
  have restrictions on num_layers, num_units, and input_size,
  though they all should be scalars.
  This fix adds the shape check of num_layers, num_units, and input_size
  for CudnnRNNParamsSize.
diff: @@ -52,6 +52,12 @@ REGISTER_OP("CudnnRNNParamsSize")
  .Attr("seed2: int = 0")
  .Output("params_size: S")
  .SetShapeFn([](InferenceContext* c) {
  + ShapeHandle unused;
  + // num_layers, num_units, and input_size should be scalars.
  + TF_RETURN_IF_ERROR(c->WithRank(c->input(0), 0, &unused));
  + TF_RETURN_IF_ERROR(c->WithRank(c->input(1), 0, &unused));
  + TF_RETURN_IF_ERROR(c->WithRank(c->input(2), 0, &unused));
  +
  c->set_output(0, c->Vector(1));
  return Status::OK();
  });
  