message: Add a field in HloInstruction to indicate whether the instruction is explicitly assigned with a default backend configuration or not.
diff: @@ -1286,6 +1286,9 @@ class HloInstruction {
  backend_config_ = std::move(config_str);
  }
  + bool is_default_config() const { return is_default_config_; }
  + void set_default_config() { is_default_config_ = true; }
  +
  // Returns a string representation of a proto in the format used by
  // raw_backend_config_string.
  //
  @@ -1734,6 +1737,10 @@ class HloInstruction {
  // HLO. See the documentation on backend_config().
  string backend_config_;
  + // This field is assigned to true when backend_config_ is assigned to
  + // a default configuration.
  + bool is_default_config_ = false;
  +
  // String identifier for instruction.
  string name_;
  