[iOS] Add watchOS support (#33318)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33318

### Summary

Recently, we have a [discussion](https://discuss.pytorch.org/t/libtorch-on-watchos/69073/14) in the forum about watchOS. This PR adds the support for building watchOS  libraries.

### Test Plan

- `BUILD_PYTORCH_MOBILE=1 IOS_PLATFORM=WATCHOS ./scripts/build_ios.sh`

Test Plan: Imported from OSS

Differential Revision: D19896534

Pulled By: xta0

fbshipit-source-id: 7b9286475e895d9fefd998246e7090ac92c4c9b6

CMakeLists.txt

@@ -343,7 +343,6 @@ if (INTERN_BUILD_MOBILE AND NOT BUILD_CAFFE2_MOBILE)
   set(FEATURE_TORCH_MOBILE ON)
   set(NO_API ON)
   set(USE_FBGEMM OFF)
-  set(USE_PYTORCH_QNNPACK ON)
   set(USE_QNNPACK OFF)
   set(INTERN_DISABLE_ONNX ON)
   set(INTERN_DISABLE_AUTOGRAD ON)

cmake/Summary.cmake

@@ -122,6 +122,7 @@ function (caffe2_print_configuration_summary)
   message(STATUS "  USE_TBB               : ${USE_TBB}")
   message(STATUS "  USE_PROF              : ${USE_PROF}")
   message(STATUS "  USE_QNNPACK           : ${USE_QNNPACK}")
+  message(STATUS "  USE_PYTORCH_QNNPACK   : ${USE_PYTORCH_QNNPACK}")
   message(STATUS "  USE_REDIS             : ${USE_REDIS}")
   message(STATUS "  USE_ROCKSDB           : ${USE_ROCKSDB}")
   message(STATUS "  USE_ZMQ               : ${USE_ZMQ}")

cmake/iOS.cmake

@@ -162,7 +162,7 @@ if (IOS_PLATFORM STREQUAL "OS")
 elseif (IOS_PLATFORM STREQUAL "SIMULATOR")
     set (DEFAULT_IOS_ARCH "x86_64")
 elseif (IOS_PLATFORM STREQUAL "WATCHOS")
-    set (DEFAULT_IOS_ARCH "armv7k")
+    set (DEFAULT_IOS_ARCH "armv7k;arm64_32")
 endif ()
 
 set (IOS_ARCH ${DEFAULT_IOS_ARCH} CACHE string  "Build architecture for iOS")

scripts/build_ios.sh

@@ -64,6 +64,12 @@ if [ -n "${IOS_PLATFORM:-}" ]; then
   if [ "${IOS_PLATFORM}" == "SIMULATOR" ]; then
       # iOS Simulator build is not supported by NNPACK
       CMAKE_ARGS+=("-DUSE_NNPACK=OFF")
+  elif [ "${IOS_PLATFORM}" == "WATCHOS" ]; then
+      # enable bitcode by default for watchos
+      CMAKE_ARGS+=("-DCMAKE_C_FLAGS=-fembed-bitcode")
+      CMAKE_ARGS+=("-DCMAKE_CXX_FLAGS=-fembed-bitcode")
+      # disable the QNNPACK
+      CMAKE_ARGS+=("-DUSE_PYTORCH_QNNPACK=OFF")
   fi
 else
   # IOS_PLATFORM is not set, default to OS, which builds iOS.