message: Add comment for Shard function
  To get better cache locality, use Shard instead of Eigen expression.
diff: @@ -364,6 +364,10 @@ struct ApplyAdamNonCuda {
  Eigen::TensorOpCost::AddCost<T>() * 5 +
  Eigen::TensorOpCost::MulCost<T>() * 6 +
  Eigen::TensorOpCost::DivCost<T>();
  +
  + // Eigen device must update 3 variables with 3 different expressions,
  + // which is bad for cache locality on CPU. Here use Shard instead of
  + // "regular" tensor expressions to get better performance.
  auto worker_threads = *(ctx->device()->tensorflow_cpu_worker_threads());
  Shard(worker_threads.num_threads, worker_threads.workers, length, cost,
  shard);
  