message: Check whether any GPUs are available in nccl_manager_test.
diff: @@ -71,6 +71,8 @@ class NcclManagerTest : public ::testing::Test {
  LOG(INFO) << "Running test with " << devices_->size() << " gpus";
  }
  + void SetUp() override { ASSERT_GT(devices_->size(), 0) << "No GPUs found"; }
  +
  static int32 NumGPUs() { return static_cast<int32>(devices_->size()); }
  static void TearDownTestCase() { delete devices_; }
  