message: [tf.data] Cache `port::MaxParallelism()` in `IteratorContext::Params::Params()`.
diff: @@ -337,7 +337,9 @@ class IteratorContext {
  if (thread_pool) {
  runner_threadpool_size = thread_pool->NumThreads();
  } else {
  - runner_threadpool_size = port::MaxParallelism();
  + static const int32 kDefaultRunnerThreadpoolSize =
  + port::MaxParallelism();
  + runner_threadpool_size = kDefaultRunnerThreadpoolSize;
  }
  // NOTE: Wrap every runner invocation in a call to Runner()->Run(), so
  