custom build script (#30144)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30144

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 git@github.com: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.

Test Plan: Imported from OSS

Differential Revision: D18612127

Pulled By: ljk53

fbshipit-source-id: fa8d5e1d3259143c7346abd1c862773be8c7e29a

tools/setup_helpers/generate_code.py

@@ -24,7 +24,8 @@ def generate_code(ninja_global=None,
                   nn_path=None,
                   install_dir=None,
                   subset=None,
-                  disable_autograd=False):
+                  disable_autograd=False,
+                  selected_op_list_path=None):
     # cwrap depends on pyyaml, so we can't import it earlier
     root = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
     sys.path.insert(0, root)
@@ -52,7 +53,8 @@ def generate_code(ninja_global=None,
             declarations_path or DECLARATIONS_PATH,
             jit_gen_dir,
             'tools/jit/templates',
-            disable_autograd=disable_autograd)
+            disable_autograd=disable_autograd,
+            selected_op_list_path=selected_op_list_path)
 
 
 def main():
@@ -71,6 +73,10 @@ def main():
         action='store_true',
         help='It can skip generating autograd related code when the flag is set',
     )
+    parser.add_argument(
+        '--selected-op-list-path',
+        help='Path to the yaml file that contains the list of operators to include for custom build.',
+    )
     options = parser.parse_args()
     generate_code(
         options.ninja_global,
@@ -79,6 +85,7 @@ def main():
         options.install_dir,
         options.subset,
         options.disable_autograd,
+        options.selected_op_list_path,
     )