message: Add comment on HloPtrComparator.
diff: @@ -1250,6 +1250,9 @@ std::ostream& operator<<(std::ostream& os, HloInstruction::FusionKind kind);
  // To make the iteration order over the map deterministic, the comparator
  // should not be using the pointer values, but rather an intrinsic property of
  // the hlo.
  +//
  +// Note that this cannot be used for HLO instructions across multiple modules
  +// since the id of HLO instructions are only unique within each HLO module.
  struct HloPtrComparator {
  bool operator()(const HloInstruction* const& lhs,
  const HloInstruction* const& rhs) const {
  