introduce INTERN_DISABLE_AUTOGRAD flag to create inference only library for mobile
Summary:
This is the first of a series of changes to reduce build size by cutting
autograd functions from mobile build.
When INTERN_DISABLE_AUTOGRAD is set:
* On CMake side we exclude Functions.h/cpp, VariableType*.h/cpp,
VariableTypeManual.cpp from the build process. Still keep variable_factories.h
as we rely on it to create variables instead of tensors.
* In source code we gate a couple autograd references (in autograd/variable.cpp)
with C10_MOBILE (technically we should use a dedicated c macro but its
maintenance cost is higher than cmake macro as we have several build systems
to change).
* Pass --disable-autograd flag to codegen script, which will stop generating
Functions/VariableType code. And for variable_factories.h it will stop
generating tracing code.
Edit: in this diff we will keep Functions.h/cpp to avoid changing source code.
Why we need this change if it's already not calling VariableType and autograd
stuff with USE_STATIC_DISPATCH=ON for mobile?
It's trying to reduce static library size for iOS build, for which it's
relatively harder to strip size with linker approach.
Why we need make involved change into codegen script?
There isn't a global config system in codegen - autograd/env.py provides similar
functionality but it says not adding anything there.
Test Plan:
- will check CI;
- test mobile build in sample app;
Differential Revision: D17202733
Pulled By: ljk53
fbshipit-source-id: 5701c6639b39ce58aba9bf5489a08d30d1dcd299
