[MPS][BE] Refactor atan2_out_mps (#107334)
It's the only function at the moment that has an int64 exception, but
check from the preprocessor define unnecessarily applied to all binary functions
Also, rename `atan2_mps_out` to `atan2_out_mps` to match the common pattern
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107334
Approved by: https://github.com/kulinseth, https://github.com/albanD
