static string CollectiveKey(OpKernelContext* ctx, int32_t group_key,
                            int32_t instance_key) {
  return strings::StrCat(group_key, ":", instance_key, ":",
                         ctx->frame_iter().frame_id, ":",
                         ctx->frame_iter().iter_id);
}
static string CollectiveKey(OpKernelContext* ctx, int32_t group_key,
                            int32_t instance_key) {
  return strings::StrCat(group_key, ":", instance_key, ":",
                         ctx->frame_iter().frame_id, ":",
                         ctx->frame_iter().iter_id);
}