message: Always gives a dummy root instruction when building a computation.
diff: @@ -240,6 +240,7 @@ void HorizontalFusionImpl::FusionCandidates::Initialize(
  VLOG(2) << "Reject non-row-major fusion instr " << instr->ToString();
  continue;
  } else {
  + VLOG(2) << "Find a fusion candidate " << instr->ToString();
  fusion_instrs_.push_back(instr);
  }
  }
  @@ -340,7 +341,12 @@ Status HorizontalFusionImpl::CreateFusedComputation(
  bound_operands->push_back(bound_opnd);
  }
  }
  - *uniq_computation = b.Build();
  + // Always create a dummy tuple instruction to serve as the root of the
  + // computation, as the existence of a root instruction is required by the
  + // HloComputation. The real root instruction will be provided below.
  + auto dummy_root = b.AddInstruction(
  + HloInstruction::CreateTuple(std::vector<HloInstruction*>{}));
  + *uniq_computation = b.Build(dummy_root);
  auto* comp = uniq_computation->get();
  // Preparing clone_map, which maps old operand to new operand.
  