message: Any unrecognised fusion type should be described as kCustom
diff: @@ -2381,7 +2381,7 @@ string ToString(HloInstruction::FusionKind kind) {
  return "kConvBackwardFilter";
  case HloInstruction::FusionKind::kConvBackwardInput:
  return "kConvBackwardInput";
  - case HloInstruction::FusionKind::kCustom:
  + default:
  return "kCustom";
  }
  }
  