message: Remove output number restriction in flatbuffer importer
  CallOnce op and other resource operators can have zero output.
diff: @@ -653,11 +653,6 @@ StatusOr<Operation*> ConvertOp(
  llvm::SmallVector<Value, 4> operands;
  llvm::SmallVector<mlir::Type, 2> outputTypes;
  - if (op.outputs.empty()) {
  - auto err = errors::InvalidArgument("operator with no outputs");
  - return emitError(loc, err.ToString()), err;
  - }
  -
  const tflite::OperatorCodeT& op_code = *op_codes.at(op.opcode_index);
  TF_ASSIGN_OR_RETURN(const std::string op_name, GetMlirOpName(op, op_code));
  