message: Return error in IdentityN shape inference function if any input shape is unknown.
diff: @@ -1501,6 +1501,14 @@ REGISTER_OP("IdentityN")
  std::vector<ShapeHandle> input;
  TF_RETURN_IF_ERROR(c->input("input", &input));
  TF_RETURN_IF_ERROR(c->set_output("output", input));
  + // If any of the input shapes are not known, we should return error.
  + for (int i = 0; i < input.size(); i++) {
  + if (!input[i].Handle()) {
  + return errors::InvalidArgument(absl::StrCat(
  + "Cannot infer output shape #", i,
  + " for IdentityN node because input shape #", i, " is unknown."));
  + }
  + }
  return Status::OK();
  });
  