Split module.cpp and export.cpp to support saving on mobile (#29881)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29881
Breaking these into separate files allows us to have three different builds:
- Mobile inference-only.
- Mobile with module saving.
- Server with module saving and other export functions like ONNX.
And this can be accomplished just by selecting which cpp files to compile,
without setting any preprocessor flags.
Test Plan: CI. Local mobile+saving build.
Reviewed By: smessmer
Differential Revision: D18509296
fbshipit-source-id: 9438273bac4624df5c7f035b2bacb901cce43053
