message: Surface the number of threads used by ctx->runner()
diff: @@ -262,9 +262,7 @@ class MapAndBatchDatasetOp : public UnaryDatasetOpKernel {
  Status Initialize(IteratorContext* ctx) override {
  mutex_lock l(*mu_);
  if (num_parallel_calls_->value == kAutoTune) {
  - // TODO(jsimsa): Surface the number of threads used by `ctx->runner()`
  - // and use it here for the default.
  - num_parallel_calls_->value = port::NumSchedulableCPUs();
  + num_parallel_calls_->value = ctx->runner_threadpool_size();
  num_parallel_calls_->tunable = true;
  }
  TF_RETURN_IF_ERROR(
  