Add test to cross function
Summary: function `cross_kernel_scalar` is not covered in `Aten/native/cpu/CrossKernel.cpp`, add tests to cover it
Test Plan:
1. Test locally to check new lines are covered
2. CI
https://pxl.cl/1fZjG
Reviewed By: malfet
Differential Revision: D22834122
fbshipit-source-id: 0d50f3a3e6aee52cb6fdee2b9f5883f542c7b6e2
