message: Make 'tensor_name' optional for HLO instructions created by hand
  Reviewers: #tensorflow, georgep
diff: @@ -72,8 +72,8 @@ static HloPoplarInstructionFactory print_tensor_factory(
  [](HloCustomCallInstruction* call)
  -> StatusOr<std::unique_ptr<HloInstruction>> {
  auto attribute_map = IPUCustomKernelsUtil::AttributeMap(call);
  - TF_ASSIGN_OR_RETURN(std::string tensor_name,
  - attribute_map.GetAttributeAsString("tensor_name"));
  + auto attribute = attribute_map.GetAttributeAsString("tensor_name");
  + std::string tensor_name = attribute.ok() ? attribute.ValueOrDie() : "";
  return CreateHloPrintTensor(call->mutable_operand(0), tensor_name);
  });
  