move XNNPACK buck build to shared build file (#77941)
Summary:
This diff moved the XNNPACK buck build to a shared build file in xplat/caffe2/third_party, so it can be reused by OSS buck CI in the future. There's no functionality change.
**Background**: as we are moving to github-first, we want community to receive more signals from our internal build. XNNPACK is part of pytorch mobile build so we want to add it to OSS BUCK CI.
**How it works**: all XNNPACK targets are defined in xplat/caffe2/third_party/xnnpack_defs.bzl. When we build it internally, the XNNPACK source is still at xplat/third-party/XNNPACK and we will load that bzl file in xplat/third-party/XNNPACK/BUCK. Everything should work as before. In OSS build, XNNPACK is a submodule in xplat/caffe2/third_party and we will load the same bzl file in pytorch/third_party/BUILD.buck.
**Wrapper Generation**: the wrapper generation script is moved to xplat/caffe2/third_party/generate-xnnpack-wrappers.py. It will take an optional argument for the path of XNNPACK (they are different in internal build and OSS build). The wrapper files will always be generated at the parent folder of XNNPACK source. But the src_defs.bzl and wrapper_defs.bzl will always be in xplat/caffe2/third_party/ (they are now called xnnpack_src_defs.bzl and xnnpack_wrapper_defs.bzl). For OSS build this script will only be used in CI, and the generated files will not be committed.
**Next Steps:** Once landed, I will try to build XNNPACK in OSS BUCK using xnnpack_defs.bzl. Meta-specific symbols need to be resolved, so there will be some refactors to the build file.
Test Plan: buck build xplat/third-party/XNNPACK:XNNPACK
Differential Revision: D36529332
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77941
Approved by: https://github.com/malfet, https://github.com/seemethere
