Change cmake to allow building with MLC kick-off build (#51326)
Summary:
- Allows build process to build with MLC enabled if subrepo folder mlc is in path and we can link against ML Compute on macOS BigSur
- To build with MLC enabled you will need to clone the mlc repo inside the pytorch repository.
- We need both this change and https://github.com/pytorch/pytorch/pull/50634 on pytorch/pytorch to enable the `mlc` device.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51326
Reviewed By: glaringlee
Differential Revision: D26533138
Pulled By: malfet
fbshipit-source-id: 0baa06b4eb2d62dbfc0f6fc922096cb0db1cc7d1
