add transformer_mean for column MEAN reduction - enables timestamp MEAN

cpp/include/cudf/detail/reduction_operators.cuh

@@ -191,12 +191,33 @@ struct compound_op : public simple_op<Derived> {
 // computes the final ResultType from the IntermediateType.
 // intermediate::compute_result method is enforced by CRTP base class compound_op
 
+template <typename T, typename Enable = void>
+class transformer_mean;
+template <typename T>
+class transformer_mean<T, typename std::enable_if_t<!cudf::is_timestamp<T>()>> {
+ public:
+  template <typename InputType>
+  CUDA_HOST_DEVICE_CALLABLE auto operator()(InputType const& t) const
+  {
+    return thrust::identity<T>{}(t);
+  }
+};
+template <typename T>
+class transformer_mean<T, typename std::enable_if_t<cudf::is_timestamp<T>()>> {
+ public:
+  template <typename InputType>
+  CUDA_HOST_DEVICE_CALLABLE auto operator()(InputType const& t) const
+  {
+    return thrust::identity<T>{}(t).time_since_epoch();
+  }
+};
+
 // operator for `mean`
 struct mean : public compound_op<mean> {
   using op = cudf::DeviceSum;
 
   template <typename ResultType>
-  using transformer = thrust::identity<ResultType>;
+  using transformer = transformer_mean<ResultType>;
 
   template <typename ResultType>
   struct intermediate {