[MPS] Fix `layer_norm_backward_mps` key (#100295)
Followup after https://github.com/pytorch/pytorch/pull/98794
See report in https://github.com/pytorch/pytorch/issues/98602#issuecomment-1527312211 and reproducer in https://github.com/pytorch/pytorch/issues/98602#issuecomment-1528214175
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100295
Approved by: https://github.com/kit1980, https://github.com/izaitsevfb
