Summary:
Create script to produce libtorch that only contains ops needed by specific
models. Developers can use this workflow to further optimize mobile build size.

Need keep a dummy stub for unused (stripped) ops because some JIT side
logic requires certain function schemas to be existed in the JIT op
registry.

Test Steps:
1. Build "dump_operator_names" binary and use it to dump root ops needed
by a specific model:
```
build/bin/dump_operator_names --model=mobilenetv2.pk --output=mobilenetv2.yaml
```

2. The MobileNetV2 model should use the following ops:
```
- aten::t
- aten::dropout
- aten::mean.dim
- aten::add.Tensor
- prim::ListConstruct
- aten::addmm
- aten::_convolution
- aten::batch_norm
- aten::hardtanh_
- aten::mm
```
NOTE that for some reason it outputs "aten::addmm" but actually uses "aten::mm".
You need fix it manually for now.

3. Run custom build script locally (use Android as an example):
```
SELECTED_OP_LIST=mobilenetv2.yaml scripts/build_pytorch_android.sh armeabi-v7a
```

4. Checkout demo app that uses locally built library instead of
downloading from jcenter repo:
```
git clone --single-branch --branch custom_build [email protected]:ljk53/android-demo-app.git
```

5. Copy locally built libraries to demo app folder:
```
find ${HOME}/src/pytorch/android -name '*.aar' -exec cp {} ${HOME}/src/android-demo-app/HelloWorldApp/app/libs/ \;
```

6. Build demo app with locally built libtorch:
```
cd ${HOME}/src/android-demo-app/HelloWorldApp
./gradlew clean && ./gradlew assembleDebug
```

7. Install and run the demo app.

In-APK arm-v7 libpytorch_jni.so build size reduced from 5.5M to 2.9M.

ghstack-source-id: ac089ad
Pull Request resolved: #30144

… custom build

Summary:
PR #30144 introduced custom build script to tailor build to specific
models. It requires a list of all potentially used ops at build time.

Some JIT optimization passes can transform the IR by replacing
operators, e.g. decompose pass can replace aten::addmm with aten::mm if
coefficients are 1s.

Disabling optimization pass can ensure that the list of ops we dump from
the model is the list of ops that are needed.

Test Plan:
- rerun the test on PR #30144 to verify the raw list without aten::mm works.

[ghstack-poisoned]

… custom build

Summary:
PR #30144 introduced custom build script to tailor build to specific
models. It requires a list of all potentially used ops at build time.

Some JIT optimization passes can transform the IR by replacing
operators, e.g. decompose pass can replace aten::addmm with aten::mm if
coefficients are 1s.

Disabling optimization pass can ensure that the list of ops we dump from
the model is the list of ops that are needed.

Test Plan:
- rerun the test on PR #30144 to verify the raw list without aten::mm works.

ghstack-source-id: 28e4a40
Pull Request resolved: #30285

)

Summary:
Pull Request resolved: #30285

PR #30144 introduced custom build script to tailor build to specific
models. It requires a list of all potentially used ops at build time.

Some JIT optimization passes can transform the IR by replacing
operators, e.g. decompose pass can replace aten::addmm with aten::mm if
coefficients are 1s.

Disabling optimization pass can ensure that the list of ops we dump from
the model is the list of ops that are needed.

Test Plan: - rerun the test on PR #30144 to verify the raw list without aten::mm works.

Differential Revision: D18652777

Pulled By: ljk53

fbshipit-source-id: 084751cb9a9ee16d8df7e743e9e5782ffd8bc4e3