Enable building of FakeLowP ops (#36170)
Summary:
We open sourced the FakeLowp ops as a reference implementation of fp16 ops. This PR makes it buildable.
```
USE_CUDA=0 USE_ROCM=0 USE_FAKELOWP=ON python setup.py install
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36170
Test Plan:
Build Onnxifi library in Glow.
```
cp ${GLOW}/build/lib/Onnxifi/libonnxifi-glow.so ${MY_PATH}/ibonnxifi.so
LD_LIBRARY_PATH=${MY_PATH}/ibonnxifi.so python pytorch/caffe2/python/fakelowp/test_sls_nnpi_fp16.py
```
It doesn't run successfully right now because we need to open source the glow gflags and some other ops like `FbgemmPack`.
Reviewed By: houseroad
Differential Revision: D20980681
Pulled By: yinghai
fbshipit-source-id: 6dd31883a985850a77261bcc527029479bbc303f
