message: Use a reader lock when reading the TfToPlatformGpuIdMap.
  We perform a look up in this map every time a kernel is executed on a GPU. A contention profile shows non-trivial contention on the current exclusive lock, which can be avoided by switching to a reader lock.
diff: @@ -59,7 +59,9 @@ class TfToPlatformGpuIdMap {
  bool Find(TfGpuId tf_gpu_id, PlatformGpuId* platform_gpu_id) const
  LOCKS_EXCLUDED(mu_) {
  - mutex_lock lock(mu_);
  + // TODO(mrry): Consider replacing this with an atomic `is_initialized` bit,
  + // to avoid writing to a shared cache line in the tf_shared_lock.
  + tf_shared_lock lock(mu_);
  auto result = id_map_.find(tf_gpu_id.value());
  if (result == id_map_.end()) return false;
  *platform_gpu_id = result->second;
  