Introducing "vectorized" byte on unpacket_traits structs

This is a preparation to a change on gebp_traits, where a new template
argument will be introduced to dictate the packet size, so it won't be
bound to the current/max packet size only anymore.

By having packet types defined early on gebp_traits, one has now to
act on packet types, not scalars anymore, for the enum values defined
on that class. One approach for reaching the vectorizable/size
properties one needs there could be getting the packet's scalar again
with unpacket_traits<>, then the size/Vectorizable enum entries from
packet_traits<>. It turns out guards like "#ifndef
EIGEN_VECTORIZE_AVX512" at AVX/PacketMath.h will hide smaller packet
variations of packet_traits<> for some types (and it makes sense to
keep that). In other words, one can't go back to the scalar and create
a new PacketType, as this will always lead to the maximum packet type
for the architecture.

The less costly/invasive solution for that, thus, is to add the
vectorizable info on every unpacket_traits struct as well.

Eigen/src/Core/arch/SYCL/InteropHeaders.h

@@ -88,7 +88,7 @@ SYCL_ARITHMETIC(cl::sycl::cl_double2)
 #define SYCL_UNPACKET_TRAITS(packet_type, unpacket_type, lengths)\
 template<> struct unpacket_traits<packet_type>  {\
   typedef unpacket_type  type;\
-  enum {size=lengths, alignment=Aligned16};\
+  enum {size=lengths, alignment=Aligned16, vectorizable=true};\
   typedef packet_type half;\
 };
 SYCL_UNPACKET_TRAITS(cl::sycl::cl_float4, float, 4)