expose USE_STATIC_DISPATCH macro to public headers
Summary:
USE_STATIC_DISPATCH needs to be exposed as we don't hide header files
containing it for iOS (yet). Otherwise it's error-prone to request all
external projects to set the macro correctly on their own.
Also remove redundant USE_STATIC_DISPATCH definition from other places.
Test Plan:
- build android gradle to confirm linker can still strip out dead code;
- integrate with demo app to confirm inference can run without problem;
Differential Revision: D17484260
Pulled By: ljk53
fbshipit-source-id: 653f597acb2583761b723eff8026d77518007533
