build torch for libtorch mobile build (#21234)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21234
ghimport-source-id: 8d401691a811991c79acf5e09e60389910910365
Differential Revision: D15616540
Pulled By: ljk53
fbshipit-source-id: 150e706630911bf14c55f47f4058eaada1edf1cc
