message: Handle optional tensors when undoing fp16 delegation
diff: @@ -1516,6 +1516,7 @@ TfLiteStatus Subgraph::UndoAllDelegates() {
  if (reg.builtin_code == kTfLiteBuiltinDequantize) continue;
  for (int i = 0; i < node.inputs->size; ++i) {
  const int original_input_idx = node.inputs->data[i];
  + if (original_input_idx == kTfLiteOptionalTensor) continue;
  if (tensors_[original_input_idx].type == kTfLiteFloat16) {
  node.inputs->data[i] = fp16_to_fp32[original_input_idx];
  }
  