message: [XLA] Change backend_config from string to bytes
diff: @@ -171,7 +171,7 @@ message HloInstructionProto {
  xla.OpSharding sharding = 40;
  // Backend configuration for the instruction. Has backend-specific meaning.
  - string backend_config = 43;
  + bytes backend_config = 43;
  // Cross replica op fields.
  repeated ReplicaGroup replica_groups = 49;
  