Add alignment to the TxN_t vectorized type (#792)

The recent removal of the union-based type punning in #781 caused misaligned access in some cases. This PR returns the alignment to the data type.

Authors:
  - Artem M. Chirkin (https://github.com/achirkin)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)
  - William Hicks (https://github.com/wphicks)

URL: #792

cpp/include/raft/vectorized.cuh

@@ -269,7 +269,7 @@ struct TxN_t {
   /** defines the number of 'math_t' types stored by this struct */
   static const int Ratio = veclen_;
 
-  struct {
+  struct alignas(io_t) {
     /** the vectorized data that is used for subsequent operations */
     math_t data[Ratio];
   } val;