gate module::save logic on mobile (#23415)
Summary:
This is part of the effort to shrink OSS libtorch mobile build size.
We shouldn't need Module::save function on mobile - it depends on
csrc/jit/export.cpp which then depends on ONNX. By gating these two
methods we can avoid these dependencies for libtorch mobile.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23415
ghstack-source-id: 87288228
Reviewed By: dreiss
Differential Revision: D16511143
fbshipit-source-id: fd031f91fcf9b7be54cbe1436506965af94ab537
