add new macro TORCH_MOBILE for libtorch mobile build (#19761)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19761
ghimport-source-id: 359a44594d3d5afb8102435b4eac6ab920c24ef4
Differential Revision: D15087652
Pulled By: ljk53
fbshipit-source-id: f1a79c38c9415bb3786cc4d073370b1cb807e5ce
