message: Allow skipping quantization of the bias tensor if it's optional.
diff: @@ -597,6 +597,9 @@ TfLiteStatus QuantizeBiases(ModelT* model, ErrorReporter* error_reporter) {
  continue;
  }
  for (const int bias_idx : property.biases) {
  + if (op->inputs[bias_idx] == -1 /*kOptionalTensor*/) {
  + continue;
  + }
  if (bias_idx >= op->inputs.size()) {
  error_reporter->Report(
  "Required input index %d is larger than the input length of "
  