[MPS][BE] Define `_compute_tolerances` (#121754)
Right now logic is mostly duplicated between `test_output_match` and `test_output_gradient_match`
So move tolerance definition logic into a shared `_compute_tolerances` function and
only keep differences (for example, grad checks are completely skipped for `torch.unique`) in the respective test functions.
Also, increase tolerance for `pow` and `__rpow__` only on MacOS-13.3 or older and remove GRAD xfaillist for those
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121754
Approved by: https://github.com/albanD
