[OSS] Enable Metal in PyTorch MacOS nightly builds (#63718)

[OSS] Enable Metal in PyTorch MacOS nightly builds (#63718)

Summary:
Build on https://github.com/pytorch/pytorch/pull/63825

Pull Request resolved: https://github.com/pytorch/pytorch/pull/63718

Test Plan:
1.Add `ci/binaries` label to PR, so the CI will build those nightly builds

2.Make sure the following CI jobs build with `USE_PYTORCH_METAL_EXPORT` option is `ON`:
```
ci/circleci: binary_macos_arm64_conda_3_8_cpu_nightly_build
ci/circleci: binary_macos_arm64_conda_3_9_cpu_nightly_build
ci/circleci: binary_macos_arm64_wheel_3_8_cpu_nightly_build
ci/circleci: binary_macos_arm64_wheel_3_9_cpu_nightly_build
ci/circleci: binary_macos_conda_3_6_cpu_nightly_build
ci/circleci: binary_macos_conda_3_7_cpu_nightly_build
ci/circleci: binary_macos_conda_3_8_cpu_nightly_build
ci/circleci: binary_macos_conda_3_9_cpu_nightly_build
ci/circleci: binary_macos_libtorch_3_7_cpu_nightly_build
ci/circleci: binary_macos_wheel_3_6_cpu_nightly_build
ci/circleci: binary_macos_wheel_3_7_cpu_nightly_build
ci/circleci: binary_macos_wheel_3_8_cpu_nightly_build
ci/circleci: binary_macos_wheel_3_9_cpu_nightly_build
```

3.Test `conda` and `wheel` builds locally on [HelloWorld-Metal](https://github.com/pytorch/ios-demo-app/tree/master/HelloWorld-Metal) demo with [(Prototype) Use iOS GPU in PyTorch](https://pytorch.org/tutorials/prototype/ios_gpu_workflow.html)

(1) conda
```
conda install https://15667941-65600975-gh.circle-artifacts.com/0/Users/distiller/project/final_pkgs/pytorch-1.10.0.dev20210826-py3.8_0.tar.bz2
```
(2) wheel
```
pip3 install https://15598647-65600975-gh.circle-artifacts.com/0/Users/distiller/project/final_pkgs/torch-1.10.0.dev20210824-cp38-none-macosx_10_9_x86_64.whl
```

Reviewed By: xta0

Differential Revision: D30593167

Pulled By: hanton

fbshipit-source-id: 471da204e94b29c11301c857c50501307a5f0785