message: Minor improvement on schema comment.
  We never supported "optional output". The comment is confusing.
diff: @@ -966,7 +966,7 @@ table Operator {
  // complicate map lookups.
  opcode_index:uint;
  - // Optional input and output tensors are indicated by -1.
  + // Optional input are indicated by -1.
  inputs:[int];
  outputs:[int];
  