Summary:
This adds some more functions to the C API, under a new DeviceUtils_c.h module. Resolves https://github.com/facebookresearch/faiss/issues/1414.
- `faiss_get_num_gpus`
- `faiss_gpu_profiler_start`
- `faiss_gpu_profiler_stop`
- `faiss_gpu_sync_all_devices`
The only minor issue right now is that building this requires basing it against an older version of Faiss until the building system is updated to use CMake (https://github.com/facebookresearch/faiss/issues/1390). I have provided a separate branch with the same contribution which is based against a version that works and builds OK: [`imp/c_api/add_gpu_device_utils`](https://github.com/Enet4/faiss/tree/imp/c_api/add_gpu_device_utils)
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/1613
Reviewed By: wickedfoo
Differential Revision: D25942933
Pulled By: mdouze
fbshipit-source-id: 5b73a86b0c1702dfb7b9e56bd741f72495aac2fd