[MPS] Calculate nonzero count inside nonzero op (#94442)
Calculate nonzero count directly in the nonzero op.
Additionally, synchronize before entering nonzero op to make sure all previous operations finished (output shape is allocated based on the count_nonzero count)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94442
Approved by: https://github.com/kulinseth
