[MPS] Fix torch.mps.is_available() (#78121)
By introducing `at:mps::is_available()` and changing `torch._C._is_mps_available` from property to memoizable callable
Also, if `_mtl_device` is released in MPSDevice destructor, shouldn't it be retained in the constructor
Looks like GitHubActions Mac runner does not have any Metal devices available, according to https://github.com/malfet/deleteme/runs/6560871657?check_suite_focus=true#step:3:15
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78121
Approved by: https://github.com/albanD
