When test set_affinity, don't hardcode the CPU ID (#65042)
Summary:
The setaffinity test always fails when the number of CPUs is smaller
than 3. Changed the test to be dynamically based on the number of CPUs
of the system.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65042
Reviewed By: jbschlosser
Differential Revision: D30960554
Pulled By: ejguan
fbshipit-source-id: 55ac12714b4b0964b48c3617b79a7a345d40ebce
