message: Fix build breakage due to missing static member definitions
  The static constexpr datamember declarations in the various ToDataType
  specialiations (in stream_executor/dnn.h) do not have a corresponding
  definition outside of the class.
  This results in compilation failures in debug mode
diff: @@ -22,6 +22,12 @@ limitations under the License.
  namespace stream_executor {
  namespace dnn {
  +constexpr DataType ToDataType<float>::value;
  +constexpr DataType ToDataType<double>::value;
  +constexpr DataType ToDataType<Eigen::half>::value;
  +constexpr DataType ToDataType<int8>::value;
  +constexpr DataType ToDataType<int32>::value;
  +
  uint64 AlgorithmDesc::hash() const {
  auto p = std::make_pair(algo_id(), tensor_ops_enabled());
  return absl::Hash<decltype(p)>()(p);
  