[MPS][BE] Avoid redispatch in `sign_out` (#110955)
By calling `at::mps::sign_outf` rather than `at::sign_out` that calls dispatcher again.
Also, do not copy output unnecessarily.
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### <samp>🤖 Generated by Copilot at f942e74</samp>
> _Metal tensors rise from the ashes_
> _`sign` and `sgn` unleash their flashes_
> _MPSFunctions reign supreme_
> _In the header of the metal dream_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110955
Approved by: https://github.com/kulinseth, https://github.com/albanD
