[MPS] Migrate round unary op to Metal

[malfet]

Stack from ghstack (oldest at bottom):

• -> [MPS] Migrate round unary op to Metal #161712

And actually use the right function, as torch.round doesn't use std::round, but rather std::rint, which can be easily seen by running something like

import torch
print(torch.arange(-3., 3., step=.5, device='mps').round())
print(torch.arange(-3., 3., step=.5, device='mps').cpu().round())

Before this change it printed

tensor([-3., -3., -2., -2., -1., -1.,  0.,  1.,  1.,  2.,  2.,  3.], device='mps:0')
tensor([-3., -2., -2., -2., -1., -0.,  0.,  0.,  1.,  2.,  2.,  2.])

But after this change results match

cc @voznesenskym @penguinwu @EikanWang @jgong5 @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @wenzhe-nrv @jiayisunx @ipiszy @chenyang78 @kadeng @muchulee8 @amjames @chauhang @aakhundov @coconutruben

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/161712

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[malfet]

@pytorchbot merge -f "Lint + MPS are green"

[pytorchmergebot]

Merge started

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