[mps] Do not use `malloc`/`free` in Indexing.mm (#78409)
Especially allocating just 2 int64 on heap is somewhat wasteful (and they are leaked if function returns earlier)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78409
Approved by: https://github.com/seemethere, https://github.com/kulinseth
(cherry picked from commit aefb4c9fba0edf5a71a245e4fd8f5ac1d65beeac)
