message: Change infeed look ahead to atomic
  Summary: Fixes T35936
  Reviewers: #tensorflow, simonl, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, jakeh
  Maniphest Tasks: T35936
diff: @@ -871,7 +871,7 @@ class InfeedPrefetchCallback : public poplar::StreamCallback {
  private:
  InfeedQueue* queue_;
  const uint64 num_bytes_;
  - std::size_t look_ahead_;
  + alignas(64) std::atomic<std::size_t> look_ahead_;
  };
  class NullPrefetchCallback : public poplar::StreamCallback {
  