[MPS][BE] Use `Tensor::copy_` (#105475)
Replace `const_cast<Tensor&>(x) = y;` with `x.copy_(y);`
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### <samp>🤖 Generated by Copilot at d2b7a0d</samp>
> _`copy_` not `clone`_
> _MPS backend runs faster_
> _Winter memory_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105475
Approved by: https://github.com/kulinseth
