remove pthreadpool.a from install directory (#25977)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25977
Call add_subdirectory() explicitly before NNPACK/QNNPACK with
EXCLUDE_FROM_ALL property so that pthreadpool target won't be installed
by default for libtorch mobile build.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25977
Test Plan: Imported from OSS
Differential Revision: D17312083
Pulled By: ljk53
fbshipit-source-id: 79851d0aa9402c5b9287ef4bbd8d7fd3a341497d
