This patch modifies the TensorContraction class to ensure that the kc_ field is
always a multiple of the packet_size, if the packet_size is > 8. Without this
change spatial convolutions in Tensorflow do not work properly as the code that
re-arranges the input matrices can assert if kc_ is not a multiple of the
packet_size. This leads to a unit test failure,
//tensorflow/python/kernel_tests:conv_ops_test, on AVX512 builds of tensorflow.
6f5b126e6d