Fix init_ops_test on AVX512 builds

The init_ops_tests unit test is failing on AVX512 builds due
to a hard coded packet size in eigen_contraction_kernel.h.
This commit fixes the issue by determining the appropriate
packet size using internal::packet_traits.  It maintains
the minimum packet size of 8 used in the existing code and
so should have no effect on non AVX512 builds.

Fixes: https://github.com/tensorflow/tensorflow/issues/25127



Signed-off-by: Mark Ryan <mark.d.ryan@intel.com>