add script to build mobile library with host toolchain (#26440)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26440

As we are optimizing build size for Android/iOS, it starts diverging
from default build on several build options, e.g.:
- USE_STATIC_DISPATCH=ON;
- disable autograd;
- disable protobuf;
- no caffe2 ops;
- no torch/csrc/api;
...

Create this build_mobile.sh script to 'simulate' mobile build mode
with host toolchain so that people who don't work on mobile regularly
can debug Android/iOS CI error more easily. It might also be used to
build libtorch on devices like raspberry pi natively.

Test Plan:
- run scripts/build_mobile.sh -DBUILD_BINARY=ON
- run build_mobile/bin/speed_benchmark_torch on host machine

Differential Revision: D17466580

Pulled By: ljk53

fbshipit-source-id: 7abb6b50335af5b71e58fb6d6f9c38eb74bd5781

CMakeLists.txt

@@ -281,6 +281,13 @@ if (ANDROID OR IOS)
   set(INTERN_BUILD_MOBILE ON)
 endif()
 
+# Setting `PYTORCH_BUILD_MOBILE` environment variable can force it to do mobile
+# build with host toolchain.
+if (DEFINED ENV{PYTORCH_BUILD_MOBILE})
+  set(INTERN_BUILD_MOBILE ON)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DC10_MOBILE")
+endif()
+
 # INTERN_BUILD_ATEN_OPS is used to control whether to build ATen/TH operators.
 # It's disabled for caffe2 mobile library.
 if (INTERN_BUILD_MOBILE AND BUILD_CAFFE2_MOBILE)

scripts/build_mobile.sh

@@ -0,0 +1,68 @@
+#!/bin/bash
+##############################################################################
+# Example command to build the mobile target.
+##############################################################################
+#
+# This script shows how one can build a libtorch library optimized for mobile
+# devices using host toolchain.
+
+set -e
+
+export PYTORCH_BUILD_MOBILE=1
+CAFFE2_ROOT="$( cd "$(dirname "$0")"/.. ; pwd -P)"
+
+echo "Bash: $(/bin/bash --version | head -1)"
+echo "Caffe2 path: $CAFFE2_ROOT"
+
+# Now, actually build the Android target.
+BUILD_ROOT=${BUILD_ROOT:-"$CAFFE2_ROOT/build_mobile"}
+INSTALL_PREFIX=${BUILD_ROOT}/install
+mkdir -p $BUILD_ROOT
+cd $BUILD_ROOT
+
+CMAKE_ARGS=()
+CMAKE_ARGS+=("-DBUILD_CAFFE2_MOBILE=OFF")
+CMAKE_ARGS+=("-DCMAKE_PREFIX_PATH=$(python -c 'from distutils.sysconfig import get_python_lib; print(get_python_lib())')")
+CMAKE_ARGS+=("-DPYTHON_EXECUTABLE=$(python -c 'import sys; print(sys.executable)')")
+CMAKE_ARGS+=("-DBUILD_CUSTOM_PROTOBUF=OFF")
+CMAKE_ARGS+=("-DBUILD_SHARED_LIBS=OFF")
+
+# If Ninja is installed, prefer it to Make
+if [ -x "$(command -v ninja)" ]; then
+  CMAKE_ARGS+=("-GNinja")
+fi
+
+# Disable unused dependencies
+CMAKE_ARGS+=("-DUSE_CUDA=OFF")
+CMAKE_ARGS+=("-DUSE_GFLAGS=OFF")
+CMAKE_ARGS+=("-DUSE_OPENCV=OFF")
+CMAKE_ARGS+=("-DUSE_LMDB=OFF")
+CMAKE_ARGS+=("-DUSE_LEVELDB=OFF")
+CMAKE_ARGS+=("-DUSE_MPI=OFF")
+CMAKE_ARGS+=("-DUSE_OPENMP=OFF")
+
+# Only toggle if VERBOSE=1
+if [ "${VERBOSE:-}" == '1' ]; then
+  CMAKE_ARGS+=("-DCMAKE_VERBOSE_MAKEFILE=1")
+fi
+
+# Use-specified CMake arguments go last to allow overridding defaults
+CMAKE_ARGS+=($@)
+
+cmake "$CAFFE2_ROOT" \
+    -DCMAKE_INSTALL_PREFIX=$INSTALL_PREFIX \
+    -DCMAKE_BUILD_TYPE=Release \
+    "${CMAKE_ARGS[@]}"
+
+# Cross-platform parallel build
+if [ -z "$MAX_JOBS" ]; then
+  if [ "$(uname)" == 'Darwin' ]; then
+    MAX_JOBS=$(sysctl -n hw.ncpu)
+  else
+    MAX_JOBS=$(nproc)
+  fi
+fi
+
+echo "Will install headers and libs to $INSTALL_PREFIX for further project usage."
+cmake --build . --target install -- "-j${MAX_JOBS}"
+echo "Installation completed, now you can copy the headers/libs from $INSTALL_PREFIX to your project directory."