Fix tests compilation on sdk < 14.4 (#2484)

Summary:
The delegate tests are failing compilation on macOS sdk  < 14.4,  added header check to make sure that MLComputePlan changes are not compiled on lower sdks.

Tested on macOS sdk 14.4 and sdk 14.2.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2484

Reviewed By: digantdesai

Differential Revision: D55018113

Pulled By: shoumikhin

fbshipit-source-id: d61031c9607db6c61bac3a9cc42b5c4f398b44d8