Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40199
Mobile custom selective build has already been covered by `test/mobile/custom_build/build.sh`.
It builds a CLI binary with host-toolchain and runs on host machine to
check correctness of the result.
But that custom build test doesn't cover the android/gradle build part.
And we cannot use it to measure and track the in-APK size of custom
build library.
So this PR adds the selective build test coverage for android NDK build.
Also integrate with the CI to upload the custom build size to scuba.
TODO:
Ideally it should build android/test_app and measure the in-APK size.
But the test_app hasn't been covered by any CI yet and is currently
broken, so build & measure AAR instead (which can be inaccurate as we
plan to pack C++ header files into AAR soon).
Sample result: https://fburl.com/scuba/pytorch_binary_size/skxwb1gh
```
+---------------------+-------------+-------------------+-----------+----------+
| build_mode | arch | lib | Build Num | Size |
+---------------------+-------------+-------------------+-----------+----------+
| custom-build-single | armeabi-v7a | libpytorch_jni.so | 5901579 | 3.68 MiB |
| prebuild | armeabi-v7a | libpytorch_jni.so | 5901014 | 6.23 MiB |
| prebuild | x86_64 | libpytorch_jni.so | 5901014 | 7.67 MiB |
+---------------------+-------------+-------------------+-----------+----------+
```
Test Plan: Imported from OSS
Differential Revision: D22111115
Pulled By: ljk53
fbshipit-source-id: 11d24efbc49a85f851ecd0e481d14123f405b3a9
